VALLEJO, Calif. (AP) _ An explosion rocked the Solano County courthouse today, smashing windows, damaging nearby buildings and sending tremors blocks away in the fourth incident in the region involving explosives in a week.

The blast around 3:30 a.m. blew a chunk 2-3 feet wide from a courthouse wall. About two dozen windows were broken in the courthouse and glass littered the ground around five nearby buildings.

No injuries were reported.

``It sounded like a sonic boom, but I knew it was a bomb because of everything that's been happening,'' said Teena Camarillo, 33, whose home is about 100 yards away.

Investigators said the bomb was hidden behind bushes in front of the building.

Hours later, investigators walked slowly across the courthouse lawn, looking for fragments of the device, which appeared to have been made from dynamite, said Charles Barnett, arson and bomb supervisor for the Alcohol, Tobacco and Firearms bureau in San Francisco.

He also said a caller today had warned of a ``firecracker device'' planted elsewhere in town, but no trace of it was found.

Officers found a wire trailing from the courthouse, through the bushes and across the lawn and a street to an alley. A car battery was found nearby, but it was not clear if it had been used to detonate the bomb.

Deputy David Robinson, a sheriff's spokesman, said it's ``reasonable to assume'' the wire was involved.

``It doesn't appear to be extremely sophisticated, but it was a pretty powerful device,'' Barnett said. He said using a wire to set off a bomb is fairly unusual.

Barnett also said there was a witness, but would not provide any details or discuss what the person might have seen.

About 100 people, including three judges, work in the building, which also houses the county district attorney's office and the public defender.

The building was expected to remain closed until Friday.

``We're extremely concerned about the situation but it's not going to stop us from operating our courts,'' said Chuck Ramey, executive officer for the courts.

The courthouse was evacuated Monday after a bomb threat. Dogs searched the building and found nothing, Robinson said.

Authorities have said they believe there is a connection between the earlier incidents. Nobody has taken responsibility for any of the incidents.

A bomb made of 30 sticks of dynamite and blasting caps was found in a rucksack leaning against the wall of the John F. Kennedy library on Saturday. It had not been armed to explode.

At that time, authorities evacuated the library, City Hall and Post Office buildings next door and a restaurant nearby.

A second device exploded about 12 hours later in front of a set of Wells Fargo ATM machines a few miles away. The machines were damaged, windows were shattered and a hole was punched in the bank building.

There were no injuries and no money was taken.

Investigators speculate nearby ATMs also were the intended target of the first bomb, but the attempt was aborted for some reason and the bomb abandoned.

Vallejo, a blue-collar town of about 100,000 people 40 miles northeast of San Francisco, was home to Mare Island Naval Shipyard for 142 years until it was closed last March as part of a series of base closures around the country.