An explosion at the federal courthouse here early Sunday morning shattered a glass entryway and damaged the lobby, but no injuries were reported, authorities said.
Someone placed a device, believed to be a pipe bomb, outside the main entrance of the Edward J. Schwartz Federal Courthouse downtown about 1:40 a.m., said FBI spokesman Darrell Foxworth.
The explosion destroyed a glass door, sent shrapnel into the ceiling and punched a hole in a fifth-story window of a building across the street, he said.
There was no major structural damage, he added.
"It's too early to tell if it's terrorism-related," Foxworth said. "It does not appear to be right now."
Investigators were reviewing videotape from security cameras and interviewing guards who were on duty when the bomb went off, Foxworth said.
"We have a very, very strong response," he said. He also said the bomb did not appear to be linked to any case underway at the courthouse.
Streets around the building remained closed as federal agents marked the location of shrapnel and took photos. Some debris was scattered over a block-long area.
The courthouse is several blocks from hotels, bars and other businesses where people might have been gathered after midnight.
The courthouse will be closed to the public Monday, Foxworth said.
Times staff writer Tony Barboza contributed to this report.