The U.S. Postal Service’s mail carriers, known for trudging through snow and ice to deliver mail, are increasingly dealing with another hazard on their delivery routes: armed robberies. 

Mail carriers, who are unarmed, are a growing target because they sometimes carry personal checks or prescription drugs, which criminals can convert into cash. Some criminals also rob carriers to get their hands on the USPS’ antiquated “arrow keys,” a type of universal key that can open many types of mailboxes, allowing thieves to steal their contents.

Last May, the USPS created a crime prevention effort called Project Safe Delivery to “reduce criminal acts against postal employees.” Even so, postal carrier robberies climbed 30% to 643 incidents last year, while the number of robberies resulting in injuries doubled to 61 in 2023, according to figures provided by the U.S. Postal Inspection Service in response to a Freedom of Information Act request by The Associated Press.

All told, robberies grew sixfold over the past decade, while the number of postal carriers held at gunpoint increased at an even higher rate, according to an analysis of the postal data.

Most recently, a gunman on Tuesday robbed a mail carrier in a New Hampshire town, with neighbors telling CBS Boston that the carrier said the robber was after his arrow keys. Police later arrested an 18-year-old man from Lowell, Massachusetts and charged him with robbery. 

That case followed multiple robberies targeting mail carriers in recent months, including:

In many cases, the carriers were not harmed, but the postal carrier in Union City suffered minor injuries that were treated at a local hospital. In some cases, postal officials are offering rewards of up to $150,000 for information to find the criminals. 

The USPS didn’t immediately respond to a request for comment Wednesday, nor did the National Association of Letter Carriers, the union that represents USPS mail carriers.

Project Safe Delivery was designed to curb mail theft and attacks on carriers, partly by replacing old locks that could be opened with arrows keys with electronic locks. But a recent CBS News review found that the postal service isn’t consistently taking steps to secure millions of arrow keys, which could be fueling the problem of rising theft.

Still, law enforcement authorities have made more than 1,200 arrests for mail thefts and letter carrier robberies since Project Safe Delivery kicked off last May. And efforts to crack down on crime targeting carriers may be paying off. As of March, postal robberies had fallen 19% over the previous five months, according to postal service data. 

—The Associated Press contributed to this report.