Boston Police are investigating a rash of thefts from US Postal Service boxes in Peabody Square, with residents reporting that mail, including checks, had been stolen, and, in one case, cashed for thousands of dollars.

Jenn Cartee, executive director of Greater Ashmont Main Streets, alerted Boston Police to the situation in last weekend after she heard of several instances of neighbors who said their mail never arrived at its posted destination.

To date, at least 11 people around Ashmont Hill, Ashmont-Adams, St. Mark’s, and Cedar Grove have weighed in, most of them after mid-September. Some victims had multiple items missing, Cartee said. They were all deposited in postal boxes in the Peabody Square area, she added.

“They drop in envelopes stamped to pay bills, or cards with gifts in them,” she said. “Some time later, they realized their stuff never got to where they were going, and in some cases the checks have been cashed.”

Last week, Capt. Tim Connolly, commander at Area C-11, said that he believes the incidents are likely a “mail fishing scam,” in which the perpetrator inserts some form of catching device into the mail slot after the last delivery and retrieves their haul overnight.

“I would call it a rash, but it’s not something new,” Connolly said. “It just popped up, and it’s the first I’m seeing of it, but it’s not a new scam; it is something that’s been going on for over a century.”

Mail theft is a federal crime, with penalties up to five years in federal prison and fines of up to $250,000.

In one instance, area resident Jim Mullahy, a onetime Post Office letter carrier, thought he was being appropriately cautious when he dropped an envelope containing a check for about $4,000 into the blue US postal box near his home on Wed., Aug. 24. He and his partner are renovating their house, and the check was made out to their architect.

He checked that the letter went fully into the box, noting that he was three hours past the day’s final pickup time of 1 p.m. No problem, Mullahy thought; it will probably be picked up and sent off before the weekend.

“It wasn’t until Sept. 1,” he said, “that I heard from the bank. In the meantime, I had no idea whether they got it.”

Citizens Bank alerted the architect, who had an account with the bank, that a check intended for him was deposited on Aug. 25 into a different account. Mullahy and his partner canceled the check, but the $4,000 had already been withdrawn from their account. Capt. Connolly said the check had been processed with no endorsement signature for an account holder whose name did not match the intended recipient’s. “Why they would then give it to the account holder or not pull it back from account holder is kind of beyond me,” Mullahy said.

While waiting for the bank to sort things out, Mullahy repaid the initial amount to the architect, leaving him for the time being out $8,000 with no clear expectation for restitution of the original $4,000.

Cartee, who has been monitoring local neighborhood groups on social media for similar reports, said that although the checks that have been illegally deposited were mostly intended for private individuals, one woman had her homeowner’s insurance check stolen and cashed.

Rose DaCorta mailed a $300 check for her insurance in August, and although it showed up on her statement as having been cashed, the insurance company sent her a notice cancellation. “The agent directed me to my bank and it was discovered that the check was cashed at a Citizens Bank,” DaCorta wrote in an email, “and I assume it was done by an individual. I don’t know if other checks were stolen because they appear on my statement as cashed.”

She and other victims used a postal box on Ashmont Street near the T station.

“It’s not clear what’s failing on the bank’s side,” Cartee said. “A number of checks were cashed without any endorsement signature, and some people have multiple checks missing with only one cashed.”

Last Thursday, a spokesman for Citizens Bank told the Reporter that its officers were looking into the Mullahy case. This week, on Wednesday, the bank issued a statement to the newspaper, saying, “The safety and security of our customers and colleagues is of utmost importance to us. For security reasons we do not disclose details about our procedures but we are working closely with the Boston Police Department and defer to them for further comment.”

For her part, DaCorta said that a woman at Citizens Bank told her she would be refunded her money within 90 days.

Cartee estimates that about $15,000 could have gone missing through these incidents. Mullahy’s $4,000 is the largest amount so far reported stolen, Connolly said last Thursday, adding that the bank was working to retrieve the ATM photo footage, which they hope will be clear enough to identify the culprit. “This is an active investigation,” with no suspects at this time, the captain said that day, and that remains the case this week, according to police.

While local police, US Postal Service inspectors, and the bank investigators are looking into the situation, Connolly said, people who know their mail has been stolen should call his department. He also recommended that people make sure their mail is dropped into the boxes before the last daily pickup time, not leaving it in over the weekend, and considering sending larger amounts of money directly at a post office.