The device, in a small microwave oven, was spotted at about 7:20 a.m. on the sidewalk next to the Greyhound bus station after the race leaders had finished the course Sunday morning, police Lt. Kevin Kraus said.
The bomb squad determined there were contents that resembled an explosive, and the area was evacuated, Kraus said. He declined to describe the materials that concerned authorities pending a further evaluation.
"Certainly, from what we did see on the X-rays, we were highly concerned," he said.
CBS Station KDKA correspondent Bob Allen reports that one officer at the scene, at a Greyhound bus station, said that the device contained a pipe bomb inside.
However, Police Chief Nate Harper would only confirm that there were suspicious contents inside the microwave.
The device has been detonated.
The disruption came a day after a car-bomb scare in New York emptied Times Square, clearing thousands of tourists from the streets for 10 hours.
Pittsburgh authorities at first tried to reroute the marathon but were unable to do so given the flow of the 26.2-mile race, Kraus said. They eventually ended up delaying the race in the area for 10 to 12 minutes, and the competition resumed after the bomb squad used a robot to disable the device and the area was cleared shortly before 11 a.m.
"At this point, we believe it was not an actual explosive device, but we are still evaluating the microwave and its contents," Kraus said. He said surveillance cameras in the area were being examined to try to find out who put the device there.
About 5,000 people took part in the full marathon.