A smoldering fire that shut down a concourse at Pittsburgh International Airport was caused by someone smoking in the boy's room.

Spokeswoman JoAnn Jenny says 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.

Jenny says investigators believe someone was habitually smoking in the rest room because they found other butts, and was using the access panel to get rid of them.

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. But it just contained a rolled up sleeping mat and authorities don't yet know how or why it got there.