One day in January, a pair of thieves in black ski masks drove into a restricted area behind a post office in Phoenix. They jumped out of their car, snatched several trays of mail that had just been dropped off and sped away.

Over six days in early April, thieves in New York City stole several bags of mail from a carrier cart, swiped packages from a Postal Service truck and made off with an entire pushcart filled with mail near the famed Grand Central station.

In a city outside Seattle, mail has been stolen so many times at a sprawling apartment complex that carriers avoid leaving unemployment and Social Security checks.

“It’s accelerated to the point where it’s insane now,” Jack Huster, a post office supervisor in Federal Way, Washington, said of the region’s mail theft problem.

The U.S. Postal Service has been under fire for months for a delivery slowdown spurred by changes brought by the new postmaster general, Louis DeJoy.

But the nation’s mail system has also been plagued by a previously undisclosed problem: It has no reliable system for tracking mail theft.

The Postal Service’s law enforcement arm acknowledged the shortcoming after NBC News, prompted by anecdotal accounts of an uptick in mail theft around the country, sought and received mail theft figures through a Freedom of Information Act request.

The Postal Inspection Service data showed that mail theft reports soared by 600% over the past three years, from about 25,000 in 2017 to roughly 177,000 through Aug. 24 of this year.

But when asked to explain the apparent explosion in mail theft, the Postal Inspection Service said the figures actually reflected multiple types of customer complaints, not just those involving theft. The agency said it couldn’t provide figures on mail theft alone due to limitations in the internal system it uses to capture customer reports.

“We discovered that the mail theft complaint numbers provided to you were overstated due to the fact they included a significant number of reported customer complaints unrelated to mail theft, such as mailbox vandalism, mail delivery issues, and matters unrelated to the mail or Postal Service,” the agency said in a statement.

“The reporting system allows for an individual to label their complaint as mail theft, however, the system is not designed to automatically discern which of these are legitimate complaints of theft of U.S. mail.”

In addition to including complaints beyond mail theft, the data provided to NBC News was misleading because the 2017 figures reflected only Aug. 23 through Dec. 31, the agency said. The Postal Inspection Service said it was planning to develop a system to better track mail theft complaints.

“For fiscal year 2021, we are developing a plan to more effectively capture our internal review of these complaints in order to track and report mail theft complaint data,” the agency said.

The disclosure of the agency’s complaint data — and the revelation that mail theft is not being closely tracked — comes during a time of intense scrutiny of the Postal Service’s ability to manage and safeguard the influx of mail-in ballots expected for the 2020 presidential election.

“To me, it’s rather startling,” said Janet Lauritsen, a criminologist who has done extensive research on national law enforcement statistics. “They are supposed to investigate these issues, so how can it be possible that they do not have the data necessary to do those investigations? I can’t make sense of it.”

Lauritsen said that despite the Postal Inspection Service’s explanation, the sharp spike in complaint numbers is troubling. The data provided to NBC News, which was shared with Lauritsen, showed a rise in complaints from 99,506 in all of 2019 to more than 177,000 through the first nine months of 2020.

“The trend in the data still indicates something, and it seems that their statements are designed to minimize alarms over the state of the Postal Service,” said Lauritsen, a professor of criminology at the University of Missouri-St. Louis.

The Postal Service did not respond to requests for comment. A spokesperson for the Postal Service Office of Inspector General referred questions to the Postal Inspection Service.

NBC News initially requested 10 years’ worth of data on mail theft reports but was told the Postal Inspection Service only had records dating back three years. No further explanation was provided.

Without reliable figures, it’s difficult to say if the nation is facing an epidemic of mail theft. But interviews with postal inspectors, police officers and supervisors, as well as local police officials, suggest parts of the country are seeing spikes in the crime.

“It’s not even a question. Mail theft has gone through the roof,” said one Postal Police officer who works on the East Coast and requested anonymity because he is not authorized to speak to the media. “And that’s only what’s reported. If they steal mail that you don’t know was coming, you don’t report it.”

Mail theft reports flow into the Postal Inspection Service through a variety of sources: postal officials, local police and customers who can file online or by phone.

When asked if data was available on reports from sources other than customers, the agency declined further comment.

Several mail theft victims previously interviewed by NBC News, as well as this reporter, never received any follow-up after reporting to the Postal Inspection Service that checks they mailed in Jersey City, New Jersey, were stolen, altered and cashed.

“They said they would open an investigation and get back to me,” said Corey Eisenberg, who filed a complaint after mailing two separate checks last year that were intercepted and cashed. “But they never did and I stopped following up.”

Thieves employ an array of strategies to steal mail, experts say, largely dictated by geography and population density.

In big cities, criminals often target the blue collection boxes that dot street corners and amass large amounts of mail. In suburban and rural areas, thieves often focus on residential curbside boxes, which tend to be far easier to break into.

The police department in Minnetonka, Minnesota, released a bulletin last month warning residents about a spike in mail theft and advising them not to leave outgoing mail in their residential boxes.

Capt. Andy Gardner said the alert was sparked by a flood of reports from people who suspected their mail was stolen and from neighbors who found several pieces of opened and discarded mail in their yards or curbside boxes.

“That means the thieves had already gone through it to look for anything valuable,” Gardner said.

No arrests have been made.

In scattered cities across the country, criminals have gone directly to the source — the post offices themselves — in an effort to steal mail.

Cases of post office burglaries have been reported in South Carolina, Minnesota and Florida — just last month alone.

Mail theft is not a new crime.

But it has evolved in many parts of the country, experts say, as criminal gangs have grown more sophisticated and identity theft has grown more lucrative. In and around larger cities, thieves have been known to work with more organized criminal networks to steal, alter and cash checks and to amass personal information from stolen mail to commit identity theft.

In the Southwest, where mail theft has been rampant for years, it remains a crime largely fueled by drug addiction, according to Liz Davis, a postal inspector and public information officer based in Phoenix.

“They aren’t mastermind identity theft scammers,” Davis said. “They are usually drug-addicted people who try to find things in the mail like gift cards and cash, so they can get their next hit.”

Earlier this year, a more audacious pair of mail thieves began hitting the Phoenix area.

The thieves made off with a significant quantity of mail from the Daisy Mountain Post Office in January, snatching trays of letters from a back dock and speeding away in a silver Toyota Corolla — all in broad daylight, authorities said.

“It was very brazen, and they were getting a lot of mail in one swoop,” Davis said.

The coronavirus pandemic ushered in an uptick in mail theft, Davis said, likely because more people were experiencing financial hardship and the arrival of stimulus checks made an alluring target.

One of the suspects in the Phoenix post office caper, identified as Rodolfo Rodriguez, was arrested on Jan. 20 and charged with three counts of mail theft and four counts of robbery of mail, according to a criminal complaint filed in Nevada.

Federal prosecutors say Rodriguez stole mail from the Phoenix post office as part of a crime wave in which he and unidentified accomplices swiped mail from postal workers’ vehicles and robbed postal workers in Las Vegas, as well as stole letters from the dock of a Las Vegas post office.

Rodriguez’s lawyer did not respond to a request for comment.

In several parts of the country, the postal service has replaced the blue sidewalk collection boxes with more secure models designed to combat a practice known as “mailbox fishing,” in which thieves pull mail out of the boxes using a wire or rope attached to an object slathered in glue or a sticky rodent trap.

The new boxes have cut down on mailbox fishing, but in New York City, thieves have merely changed tactics.

No longer able to fish mail out of sidewalk boxes, they began to target the carriers themselves, as well as their trucks.

“The tamper-proof boxes just changed the mode of crime,” said James Bjork, national business agent for the Postal Police Officers Association.

NBC News obtained internal Postal Police daily incident reports from late March to early May, four days in late May, mid-July to mid-August and a couple of days in early September.

The reports document 24 cases of thieves targeting carriers or their vehicles over 83 days — on average, one every 3 1/2 days.

Some are harrowing.

While delivering mail at a building in the Bronx, New York City, on April 22, a mail carrier observed an individual going through his mail cart, a report says. As the carrier approached, the individual pointed a gun at him and fled. The individual was later arrested by New York City Police Department officers, and the weapon was found to be a paintball gun, a report says.

Two weeks earlier, on April 9, a mail carrier had his arrow key taken from him at knifepoint in Brooklyn, a report says, documenting a crime with significant consequences. Arrow keys are used to open up a variety of Postal Service boxes in a given neighborhood. Stealing one gives an assailant access to an entire universe of mail.

“When a thief gets these arrow keys, it’s huge,” said the Postal Police officer who spoke on the condition of anonymity. “They are basically skeleton keys and they open every collection box, every relay box and every cluster box in a given ZIP code.”

A Postal Service Office of Inspector General report released in late August found that the Postal Service’s management controls over arrow keys were “ineffective.”

“Specifically, the number of arrow keys in circulation is unknown, and local units did not adequately report lost, stolen, or broken keys or maintain key inventories,” the report says.

Earlier this month, Kevin Ronny Williams, of Brooklyn, pleaded guilty to stealing mail in upstate New York using a Postal Service key given to him by a rogue Postal Service employee.

Williams, 23, admitted to using the key to open mail collection boxes in Liverpool in early March, according to federal prosecutors. Williams didn’t operate in street clothes: He wore a Postal Service letter carrier uniform and clutched a Postal Service mail satchel while committing the crimes, prosecutors said.

The former Postal Service employee who allegedly helped him has also been charged and is awaiting trial, prosecutors said.

In the Seattle suburb of Federal Way, located along the Puget Sound, thieves swipe mail from curbside boxes with alarming regularity, said Huster, the post office supervisor.

He suspects the city’s location, surrounded by highways, is one factor that makes it a magnet for thieves.

“We have incredibly easy access to I-5 and the Pacific Highway,” Huster said. “You can hit a place and literally vanish.”

“Every day it happens,” he added. “The amount is ridiculous.”

Mail carriers know to avoid leaving easily identifiable government checks in the cluster boxes at a large apartment complex in Federal Way, Huster said, because mail has been stolen from them on a regular basis.

“We leave a notice informing them to pick it up at the post office,” Huster said. “This complex has really been a thorn in my side.”

Huster said the thefts in his area largely victimize people who are older and live on fixed incomes.

“These are often low-income people, someone’s grandma and grandpa,” he said. “They are the core people we serve, and I’m just left speechless. I feel awful for them.”