Baltimore's Francis Scott Key Bridge collapsed early Tuesday morning after one of its columns was hit by a large container ship. Video of the bridge shows it buckle and crash into the river below after the Dali, a massive 948-foot cargo ship, strikes it, sending vehicles and people into the river below. Six people were missing, and after a daylong search and rescue effort, the U.S. Coast Guard said Tuesday evening it was shifting to a recovery mission.

Here's what we know about the condition of the bridge and how this sort of collapse could happen.

During its latest federal inspection, the Key Bridge scored a six out of nine, which is considered "fair," a condition it had been labeled since 2008, CBS Baltimore reported. 

The bridge passed inspection in May 2022, but there was concern with one of its columns. The reinforced concrete column was downgraded from a health index, or condition rating, of 77.8 to 65.9. It is not known whether the column of concern was the same one that was struck on Tuesday morning.

Ben Schafer, professor of civil and systems engineering at Johns Hopkins University, told CBS News that most bridges in the U.S. fall in this "fair" range, which is "not what we would want or suspect" as a society. But, he said, the massive ship — not the condition of the Key Bridge — is likely to blame for its collapse. 

"Like others, I've watched the video a bunch of times and have gone frame by frame and worked up a few different hypothesis in my mind, but I see little evidence for anything other than: the boat strike took a bridge with two supports and took away one of them and then it fell in the water," Schafer told CBS News. "I mean, doesn't seem to be super more complicated than that."

Transportation Secretary Pete Buttigieg expressed a similar view at a briefing Tuesday afternoon. "This is a unique circumstance. I do not know of a bridge that has been constructed to withstand a direct impact from a vessel of this size. Anytime anything happens to any bridge, we as a country take that and learn from it," Buttigieg said.

Many bridges, whether a suspension bridge or an arched bridge, require two supports under the portion of bridge that goes over water, Schafer said. "This is the minimum number to cross over," he said. 

"The container ship was as wide as it was as tall," Schafer said, adding that it was of similar scale to the bridge. "So the mass that's associated with that container ship creates an amount of energy that a small concrete pier isn't going to sustain. That's an accident that we can't allow to happen and expect the bridges to stay up." 

Schafer said in the late 1800s, the Quebec Bridge, which had a similar design to Baltimore's Key Bridge, collapsed, which taught engineers many lessons about truss bridges. But by the 1970s when the Key Bridge was built, the design used was modern and sufficient. 

Schafer said there is a lesson to be learned here, but it's likely not that changes to a bridge's design would help prevent an accident like this. He said the way shipping traffic is managed, or the size of container ships — which have likely increased since the 1970s when the bride was built — should be considered when trying to understand the risk of these types of accidents. 

Robert Sumwalt, former chairman of the National Transportation Safety Board, told CBS News that the video of the accident holds some clues about what could have happened. The lights of the ship flashes off and on, which would indicated an electrical power failure, he said. Authorities confirmed the ship did lose power before the crash. 

Black smoke is also seen coming out of the ship, which Sumwalt said likely means "ship's crew is trying to increase the RPMs across the propeller to increase the steering capability of it."

All of the crew members on the Dali were accounted for and there were no reports of any injuries among the crew. Sumwalt also said the ship's audio recording device likely captured conversations happening on the ship as well as other data like the angle of the rudder. "There will be a lot of information to help piece this back together," he said. 

The four-lane Key Bridge is 1.6 miles long and was used by some 31,000 people a day, according CBS News Baltimore. Sumwalt said only two other tunnels cross the Patapsco River, so without the bridge, access will be limited. 

A construction crew was filling potholes on the bridge at the time of the accident. Two workers were rescued from the water but six remained unaccounted for Tuesday night, officials said. 

In:
• Maryland
• Baltimore
• Bridge Collapse

Caitlin O'Kane

Caitlin O'Kane is a New York City journalist who works on the CBS News social media team as a senior manager of content and production. She writes about a variety of topics and produces "The Uplift," CBS News' streaming show that focuses on good news.