Members of the Walnut Creek Police Department's bomb squad also detonated two bombs on Wednesday that were found about 100 yards from today's bomb, Concord police Sgt. Matt Morrissey said.
Police said that at about noon today, a man walking in the park found a cylindrical device near Turtle Creek Road and carried it to a nearby park maintenance area.
Park employees told the man put to the object down and then left the area and called Concord police.
Responding officers concluded the device was likely a pipe bomb and evacuated a large section of the park.
The Walnut Creek bomb squad came and detonated the device. Their investigators later determined it was in fact a pipe bomb and that its fuse had been ignited but failed to detonate the bomb, Concord police said.
Several local law enforcement agencies helped Concord police comb the park for other explosives this evening, but none were found, Concord police Lt. Steve Dyer said.
BART police K-9 bomb detection units, Alameda County sheriff's deputies, and members of the Contra Costa County Sheriff's Office search and rescue team assisted.
The park, which is located east of Treat Boulevard, was closed this afternoon during the search.
It was reopened at about 7 p.m., Morrissey said.
Two other pipe bombs were found in the park Wednesday afternoon and detonated by Walnut Creek police.
Morrissey said police have not determined a suspect or a motive but are actively investigating the incidents.
Residents are reminded to not touch or move any suspicious devices and should instead call police or notify a parks worker if any more are found.
Anyone with information about the pipe bombs is asked to call the Concord Police Department's detective bureau at (925) 671-3030.
