Multiple manhole fires caused a loud explosion sound near Union Square and damaged nearby buildings, including the Strand Bookstore, Friday morning, officials said.

The FDNY received multiple calls about manhole fires along 12th Street, starting around 5:30 a.m., but the loud sound was heard shortly before 8 a.m., when the manhole on Broadway and 12th Street exploded, officials said. A total of three manholes had caught fire, Con Edison said. 

The cause of the fires is under investigation, but ConEd said it was likely weather-related. The rain likely pushed salt, leftover from snowstorm preparation, into the electrical system underground, which does not mix well, a spokesman said.

Photos from the scene show some buildings, including the Strand Bookstore on the corner of Broadway and 12th Street, with broken windows after the explosions. The Department of Buildings said it was investigating the damage.

A few buildings were evacuated due to high carbon monoxide readings, ConEd said. No injuries were reported.

They fires were placed under control at about 9:20 a.m., the FDNY said.