PITTSBURGH — Lightning struck a clock tower at a historic public library, causing chunks of granite to plunge through the building's roof and damage several rooms. No injuries were reported.

The incident prompted officials to close the Carnegie Library of Pittsburgh's Allegheny Regional Branch in the city's North Side neighborhood while officials assessed the damage.

Library spokeswoman Suzanne Thinnes said the book collection was unharmed, but that the building would be off limits to patrons until further notice, possibly for weeks.

Officials planned to bring a crane to the library to remove the fallen debris.

Nearby residents said they heard what they thought was a bomb blast around 8:15 p.m. Friday, after the building had closed for the night.

The lightning bolt knocked a pyramid-shaped granite finial from the top of the tower, sending it crashing through the roof, cracking pipes in the attic and unleashing a stream of water down the stairs.

Another large chunk of granite tore through the attic and into a second-floor lecture hall, pulling down a steel beam and a section of a ventilation duct. It missed a baby grand piano by a few feet.

No library books were damaged by water, but many were coated with dust, Thinnes said.