Police are on high alert after several Bay Area officers have been recently attacked while off duty.

An internal police document obtained by NBC Bay Area shows the attacks are raising concerns about officer safety in and out of uniform. Sources confirm to NBC Bay Area three Bay Area officers have been attacked while off duty. One of the officers suffered serious injuries, according to sources.

"We all take this job knowing the risks are there," said Detective Salvador Zuno with the San Mateo County Sheriff's Office.

Zuno said his deputies were not involved in the recent after work attacks. The sheriff's office nonetheless alerted its deputies.

"It's a constant reminder to us to always be vigilant," Zuno said. "We go to our training. We're always aware of our surroundings."

Officers interviewed said the warning is important, but will not change what they do. The officers said they signed up for a career with serious risks, even if that risk sometimes follows them home.

A retired cop, who served 30 years in law enforcement and did not want to be identified, still remains on guard even today as a civilian.

"When I walk into a restaurant I'll know all the 10 people sitting there, and if I've run into them before," the cop said. "And then I'll sit down with my back to the wall."