(CNN) -- The FBI's counter-terrorism unit is trying to determine who detonated a small explosive device early Sunday at the federal courthouse in San Diego, California.
A suspected pipe bomb damaged the federal courthouse in downtown San Diego early Sunday.
The bomb, which damaged windows but caused no injuries, went off about 1:40 a.m. (4:40 a.m. ET), just outside the Edward J. Schwartz Federal Courthouse, the FBI said.
The blast shattered a glass door to the courthouse and broke a window in a building across the street, the bureau reported.
The FBI's Joint Terrorism Task Force took over the investigation from local authorities after investigators determined the blast was the result of an explosive device.
No claim of responsibility has been issued, said Special Agent Darrell Foxworth, an FBI spokesman. The streets around the building were blocked off as investigators searched the building, which is near the city's waterfront. Watch agents at the scene of the explosion »
But Foxworth told CNN he expects the courthouse to be open for business Monday.
"We're hoping that we'll be able to complete our search sometime this morning and have the streets open towards the end of the morning," he said.
Investigators believe the device was a pipe bomb, he said. Security personnel were inside the building at the time of the blast, but no one was hurt.
The FBI is checking security cameras at the courthouse and surrounding buildings for evidence, Foxworth said. E-mail to a friend
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