NORTH MIAMI (CBS4) – For the third time in less than two weeks, a postal worker has been attacked and robbed while making their rounds in north Miami.

Now police are trying to determine if the robberies are related.

The latest attack happened Monday outside an apartment building on NE 6th Avenue at 133rd Street. While on her way to the mailboxes outside the building, a female postal carrier was robbed at gun point of her master-key.

Last Friday another carrier, also female, was struck in the face by two teens who then stole her master-key. The first attack happened in Miami Gardens on October 7th; again, a female carrier was targeted.

The carriers all escaped with minor injuries but the U.S. Postal Service is alarmed by rash of attacks.

“We have revved up in terms of violence. The first carrier was grabbed and shaken up. Now a gun was displayed. This is definitely escalating,” said U.S. Postal Inspector Blad Rojo.

Last year, 60-year old carrier Bruce Parton was killed during a robbery in which someone stole his master-key along with several other items.

Nestor Gonzalez, a carrier for 17 years, told CBS4′s Peter D’Oench that he’s very concerned and keeps his guard up now more than ever.

“You’ve got to be careful on your job,” said Gonzalez. “Careful of your surroundings, people who look strange and who are following me, that’s what I look for.”

Gonzalez said after all these years he never thought he’d be looking over his shoulder while on his route.

“This is the sign of the times we live in,” said Gonzalez. “I feel sorry for those carriers. This could have been me.”

The descriptions of the attackers in the three incidents differ so police are trying to determine if they are related.

A source in law enforcement told D’Oench that investigators were trying to see if the suspects were all part of the same crime ring.

“I hope they catch them and put them away. We are out there to do good for people and not hurt people. And all this is happening for a key. It’s not worth it,” said Gonzalez.

“Letter carriers are public servants. They are already doing a dangerous job. They don’t need to add to it,” said Rojo.

The Postal Service is offering a $5000.00 reward for information that leads to a conviction in Monday’s attack. The number to call is 877-876-2455 then hit option #2.

Investigators told D’Oench that the apartment building did have a surveillance cameras but it’s not known if the videotape was good enough to release.