National Transportation Safety Board investigators spent Saturday on scene investigating the bridge that collapsed in Pittsburgh one day earlier.

Three people were hospitalized but no one was killed when the Fern Hollow Bridge collapsed before rush hour Friday. Five cars and a Port Authority bus were on the bridge when the bridge collapsed.

NTSB chairwoman Jennifer Homendy told reporters Saturday she expects the investigation to be “long” and “technical.”

“Our mission here is to determine how this collapse happened, why it happened — to prevent it from happening again,” she said.

Investigators used a drone to map the scene and did an initial inspection of the structure and the legs under the deck of the bridge, Homendy said.

“We want to see how the structure came to rest and we’re looking for things like signs of corrosion, signs of fatigue cracking,” she said.

In addition to analyzing the wreckage at the scene, Homendy said her investigators will look at any video evidence and is looking to obtain any video taken from cameras inside the bus that was on the bridge at the time of the collapse.

Homendy said they were able to obtain video from a local business that determined the time of the accident was approximately 6:40 a.m. She called on anyone with additional video to contact them to aid in the investigation.

“We’re going to put this bridge under a microscope,” Homendy said. “We’re going to look at the entire history of this bridge — from the design, construction, maintenance repair, all work up until the day of the collapse.”

Investigators will also look at the inspection frequency of the bridge, as well as federal and state procedure loads on the bridge, she said.

Homendy expects a preliminary report to be released in 10 days. While a cause of the collapse cannot be determined yet, she said the incident should “serve as a call to action.”

“Of the 618,456 bridges in the United States, 47.7% are rated fair, 7.3% are rated poor,” Homendy said. “Of the 22,965 bridges in Pennsylvania, 52.8% are rated fair and 14.6% are rated poor. So, this is a call to action because we have aging transportation infrastructure.”

When asked what it was like to see the damage up close, Homendy said it’s fortunate no one died.

“Injuries are very serious, but when I looked in the position of the bus, and the position of the vehicles, I’m really thankful that no one lost their life in this collapse,” she said.