In mid-April, 83-year-old Josefina Gomez Pando put a check for $112 into a blue United States Postal Service mailbox on a corner in the Upper West Side of Manhattan — but her check never made it to the post office.

That’s because thieves, most likely using a sticky rat trap attached to the end of a string, fished Pando’s check out of the mailbox, “washed” the check and wrote a new one for $3,500.

“I pay all my bills by mail  —  around 30 checks a month,” said Pando, who owns three buildings in New York City. “This never happened to me.”

As computers are becoming more difficult to hack, criminals have began targeting mailboxes in search of checks, gift cards and personal information useful in stealing identities.

According to law enforcement officials, mailboxes are increasingly becoming a source for criminals who are “mail fishing,” which entails using tools to retrieve envelopes out of public mailboxes.

“It’s doubled over the last two years, at least,” said Lt. John Grimpel, a spokesman for the New York City Police Department.

Mail fishers are not unique to New York City. The same crimes are being committed in Texas, Florida, New Jersey, Massachusetts, Colorado and California, among other places, CNBC reported.

“It’s coast to coast,” Grimpel said.

Grimpel noted that thieves access the mail by putting sticky rodent traps on a wire or string, feeding them through the mail slot and down into the box, and pulling up envelopes — sometimes 20 at a time.

R. Sean McCleskey, a retired United States Secret Service agent who supervised an identity theft task force for more than a decade, said that mail fishers employ a variety of techniques.

“I’ve seen more traps that I can tell you,” said McCleskey. “I’ve seen them rol up and put a chain around the mailbox and drag it down the street.”

Mail fishers often “catch” people’s gift cards, cash-filled birthday greetings, rent checks, money orders, credit cards or documents containing home addresses and Social Security numbers.

“You can basically build a profile of an individual from the information you gather in the mail,” McCleskey said.

When it comes to your checks, thieves can “wash” them of ink with easy-to-purchase chemicals, leaving them with a blank check to use however they please.

As for Pando’s check, her bank, CitiBank, noticed the fraud and did not process the $3,500 payment.

Authorities recommend several tactics to ensure the safety of your mail:

Deposit your mail prior to the last collection time of the day, preventing it from sitting in the box overnight when most mail fishers operate.

Or, McClesky advised, “Take your mail and put it in the post office, and when I say in the post office, I mean walk in and put it in, which is kind of a pain but I’ve seen mail stolen directly out of the receptacles outside as well.”

McClesky also highly recommended requesting tracking numbers for your mail and following each piece.

The New York City Police Department advises that those writing checks use a pen with permanent ink, which is more difficult to wash away.

Another way to safeguard yourself is to closely monitor your bank account frequently to make sure your checks were cleared by the establishments for which they were intended. Contact your bank if you discover an issue.

If you see someone mail fishing, call the United States Postal Inspection Service at 877-876-2455, so it can investigate.