A small explosion in a lounge area of the downtown public library Friday afternoon blew out a window on the third floor, and forced 400 people inside the building to evacuate, police said. No injuries were reported.

The remnants of a bag believed to have held the explosive were found on the third floor, near some desks and tables, police Detective Kevin Joiner said.

There were no reports of threatening phone calls to the library and "nobody has taken any responsibility at this point," Joiner said.

The block around Library Square was closed to the public as officers searched the building, police said. From the street, investigators in white protective suits could be seen taking pictures and inspecting the third floor.

Conrad Olsen, 45, who was at a computer on the third floor, said he smelled sulfur and saw a white fast-food bag blown open. But he said he saw no one.

The six-story, 240,000-square-foot library, with a swooping architectural design and more exterior windows than walls, opened in February 2003. It has a collection of more than 500,000 books and other materials.