Police received a call shortly after 5 a.m. reporting shots fired or a possible explosion in the 200 block of Norcross Lane.
When officers arrived, they found a man in the driveway with injuries to his hands, face and torso, Kollo said.
The man, who is in his 50s, told officers that he had made the pipe bomb the previous day and that it was in his hands when it exploded, Kollo said.
He was taken to John Muir Medical Center, where he was put into a medically induced coma. It was unclear Tuesday afternoon whether the man would survive.
Meanwhile, officers evacuated four families from nearby houses and called in the Walnut Creek bomb squad to make sure there weren't any more explosive devices.
The bomb squad, which serves the entire county, did not find any additional bombs.
Police have had no prior contact with the man, and found no evidence that he had an intended target for the bomb or that he made the device with anyone else, Kollo said.
"We have no reason to believe anyone else was involved," Kollo said. "It appears to be an isolated incident."
The man did not live in the house where the explosion happened, but had apparently been allowed to stay in a trailer in the driveway, Kollo said.
Nobody else was hurt.