PITTSBURGH — A smoldering fire that shut down a Pittsburgh International Airport concourse for several hours and prompted a bomb scare was caused by someone smoking in a men’s room, an airport spokeswoman said today.

Spokeswoman JoAnn Jenny said the butt caught some paper towels and other debris on fire that had been stashed behind a plumbing access panel in a men’s room. Investigators found other butts and believe someone has been habitually smoking in the restroom and using the access panel to get rid of the evidence, Jenny said.

A bomb squad was called out and one concourse shut down for more than three hours this morning because investigators also found a satchel behind the panel and feared it might be a bomb or other incendiary device. But it just contained a rolled up sleeping mat and authorities couldn’t immediately determine how or why it got there, Jenny said.

The smoke was reported about 7:30 a.m. today and investigators traced it to the plumbing access panel in a men’s bathroom within an hour.

By 11 a.m., the bomb squad determined the satchel posed no threat and the concourse was reopened.