PITTSBURGH — A 50-year-old bridge spanning a ravine collapsed in Pittsburgh early Friday, requiring rescuers to rappel nearly 150 feet (46 meters) while others formed a human chain to reach occupants of a dangling bus.

The collapse came hours before President Joe Biden was to visit the city to press for his $1 trillion infrastructure bill, which has earmarked about $1.6 billion for Pennsylvania bridge maintenance.

There were minor injuries from the collapse but no fatalities, said authorities, who also flew drones to make sure no one was under any collapsed sections.

Police reported the span, on Forbes Avenue over Fern Hollow Creek in Frick Park, came down just before 7 a.m. Witnesses said the loud noise from the collapse was followed by a hissing sound and the smell of natural gas.

“The first sound was much more intense, and kind of a rumbling, which I guess was the structure, the deck hitting the ground,” said Ken Doyno, a resident who lives four houses away. “I mean, the whole house rattled at that point.”

A spokesperson for the National Transportation Safety Board, an independent government agency that investigates transportation problems, said Friday the agency was sending a team of about 10 people to investigate “not only what happened but why it happened.”

Sam Wasserman, a spokesperson for Pittsburgh Mayor Ed Gainey, said a few hours after the collapse that officials were evaluating the scene and an urban search-and-rescue team was still combing the area for any other possible victims.

He said most of the 10 people who were evaluated for injuries were first responders checked for exhaustion or because of the cold and snowy weather. Three people were taken to hospitals and none had critical injuries, Wasserman said.

Wasserman said the two-part, elongated Port Authority bus was on the bridge when it collapsed, with a driver and at least one passenger who were both evaluated by emergency medical responders.

Neighbors said a gas company worker went door-to-door to get them to evacuate from the immediate vicinity before the gas was successfully shut off.

“Apart from just this abiding noise, we could begin to smell gas and that was the truly frightening thing, then with that smell we both said, let’s get dressed and get out of here,” said Lyn Krynski, whose home is nearest the bridge.

“It sounded like a weather phenomenon more than anything,” said Douglas Gwilym, who was shoveling about an inch of snow when he heard the noise. “It was all I had to compare it to — it was this odd, whooshing sound.”

At the site of the collapse, Lt. Gov. John Fetterman called it “just an awful, surreal scene.”

“I hope it’s a wake-up call to the nation that we need to make these infrastructure investments,” Fetterman said.

The scene was reminiscent of the aftermath of an earthquake. There was a large crack on the end where the bus was. There was also a car upside down in front of the bus.

The bridge is an important artery that leads to the Squirrel Hill and Oakland neighborhoods, and a popular route toward downtown Pittsburgh. Authorities told motorists to avoid the area. Several neighbors said a weather-prompted two-hour school delay may have prevented a far worse human tragedy.

In a statement, the White House said Biden would proceed with his planned trip to Pittsburgh.

“Our team is in touch with state and local officials on the ground as they continue to gather information about the cause of the collapse,” the statement said. “The President is grateful to the first responders who rushed to assist the drivers who were on the bridge at the time.”

The steel span, which was built in 1970, carries about 14,500 vehicles a day, according to a 2005 estimate.

Wasserman said the most recent inspection occurred in September but the report was not immediately available.

But a September 2019 inspection of the city-owned bridge revealed the deck and superstructure to be in poor condition, according to the U.S. Department of Transportation’s National Bridge Inventory. A spreadsheet on the state Department of Transportation website listed the bridge’s overall condition as poor, which, according to the Pennsylvania Department of Transportation, means “deterioration of primary structural elements has advanced.”