PITTSBURGH (AP) -- 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 Thursday.

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 Thursday 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 at about 7:30 a.m. Thursday and investigators traced it to the plumbing access panel in a men's bathroom within an hour.

Because they also found the black satchel nearby, extra precautions were taken and the county bomb squad was called, Jenny said. The D concourse, where the restroom is located, was closed and investigators attempted to determine if the satchel contained anything that might have caused charring found around the edges of the access panel, she said.

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

"There's no threat involved in any of this," Jenny said. "It's not like anybody was trying to plant a bomb or anything like that."

The D concourse is one of four lined with dozens of airline gates at the X-shaped terminal about 10 miles west of the city. Delta, United and American airlines flights that normally use the concourse were sent to gates in other parts of the airport, but there were no reports of major delays.