With mail theft and postal carrier robberies up, law enforcement officials have made more than 600 arrests since May in a crackdown launched to address crime that includes carriers being accosted at gunpoint for their antiquated universal keys, the Postal Service announced Wednesday.

Criminals are both stealing mail and targeting carriers’ so-called “arrow keys” to get access to mailboxes.

“We will continue to turn up the pressure and put potential perpetrators on notice: If you’re attacking postal employees, if you steal the mail or commit other postal crimes, postal inspectors will bring you to justice,” Chief Postal Inspector Gary Barksdale told reporters on Wednesday.

The Postal Service announcement on Wednesday came against a backdrop of rallies by the National Association of Letter Carriers calling for better protection of carriers and harsh punishment for criminals who rob them. They’ve been held across the country in recent months, including one Tuesday in Denver and another Wednesday in Houston.

“Postal carriers have to keep their head on a swivel. They have to take precautions to be extra vigilant with their surroundings,” Rick Byrne, president of the Colorado State Association of Letter Carriers, said Wednesday.

Letter carriers are on edge after nearly 500 of them were robbed last year. Criminals increasingly targeted the mail to commit financial crimes like altering checks to obtain money.

Postmaster General Louis DeJoy said in a statement that it’s important to protect the “sanctity of the nation’s mail” but that his top priority is the safety of those delivering it.

To reduce robberies, the Postal Service is in the process of replacing tens of thousands of postal carriers’ universal keys that are sought by criminals seeking to steal mail to commit check fraud, officials said. So far, 6,500 of the keys have been replaced with electronic locks in select cities, and another 42,500 are set to be deployed, officials said. The Postal Service has declined to say how many of the arrow keys are in service.

To prevent mail theft, the Postal Service also has deployed more than 10,000 high-security blue boxes in high-risk locations to prevent people from breaking into them.

The Postal Service also implemented changes that reduced fraudulent change-of-a-address transactions by 99.3% over the past fiscal year, and they’ve reduced counterfeit postage by 50%, as well, officials said.

The Postal Service is touting its successes after a critical report by its own watchdog, the Office of Inspector General. Issued late last month, it faulted management for a lack of “actionable milestones,” accountability for staffing and training, and upgrading carriers’ universal keys.

The U.S. Postal Inspection Service is leading the effort with other internal units and outside law enforcement agencies. Early efforts focused on organized mail crime in Chicago, San Francisco and several cities across Ohio.

Of the 600-plus arrests made since May as part of “Operation Safe Delivery,” more than 100 were for robberies while more than 530 were for mail theft, officials told reporters Wednesday.

The penalty is steep for interfering with the mail.

Theft alone can be punished by up to five years in prison; possession or disposal of postal property carries a sentence of up to 10 years in prison. Assaulting a mail carrier can also lead to a 10-year sentence for a first-time offense. Repeat offenders can get 25 years for an assault.