Pittsburgh ? A bomb threat led police to close the city’s three major vehicular tunnels during the evening rush hour Thursday, causing serious traffic snarls.

Nothing suspicious was found, and all three were reopened within an hour, authorities said.

A caller to 911 said about 4:45 p.m. that a bomb would explode at 6 p.m. at a tunnel on a road referred to locally as the parkway, said state police Trooper Robin Mungo.

The Fort Pitt and Squirrel Hill tunnels are on the parkway, and authorities opted to close them “to make sure everyone was safe and to continue our sweeps,” Mungo said.

Authorities later closed another tunnel when they got a report that a package had been found, said Jim Struzzi, a spokesman for the Pennsylvania Department of Transportation. The report was unfounded, he said.

The three tunnels were reopened shortly after 6:30 p.m., but not before the ordeal had caused headaches for thousands of drivers. The tunnels carry three of four main thoroughfares into and out of the city.