Story highlights City workers in Placentia found the backpack in a storm drain, a sheriff's spokesman says

6 grenades, blasting caps and fuse igniters were in the backpack and nearby dirt

The explosive devices likely had been there for some time, sheriff's spokesman adds

They have been safely detonated, he says

City workers in Southern California discovered a backpack Monday filled with grenades and other explosive devices, a sheriff's spokesman said.

The municipal employees came across the backpack in a storm drain in Placentia around 9 a.m., Orange County Sheriff's Department spokesman Jim Amormino told CNN.

Police and bomb squad personnel subsequently responded and located six grenades, blasting caps, fuse igniters and some white and black powder, according to Amormino. These were discovered in the backpack as well as buried in dirt nearby.

It appears the explosive devices were put in the area intentionally, according to the sheriff's department spokesman, who added that he believes they had been there for some time.

FBI agents were on scene Monday afternoon helping sheriff's department members in their investigation, as is routine when explosive or suspicious items are found, said Ari Dekofsky, a spokeswoman for the bureau's Los Angeles office.

There was no immediate threat to the public's safety, added Dekofsky.

By 2:45 p.m., all the explosive devices had been safely detonated, said Amormino.

Placentia is a city of about 50,000 people located just north of Anaheim and 30 miles southeast of Los Angeles.