A man who identified himself as a former Baltimore Fire Department employee said he could see the bridge from his bedroom window.
“We woke up to an earthquake and a long sound of thunder,” he told local media.
“I saw some emergency lights in the area and decided to drive over… What was going on was a multi-jurisdictional response to a disaster.”
The 1.6-mile (2.6-kilometre), four-lane bridge spans the Patapsco River southwest of Baltimore.
It opened in 1977 and carries more than 11 million vehicles per year, about 31,000 daily.
It is a major part of the road network around Baltimore, an industrial city on the US East Coast next to the capital Washington.
The Maryland Transportation Authority asked drivers to avoid the area, which is part of the I-695 interstate highway, which it called an “active scene.”
The Singapore-flagged container ship Dali had stopped under the bridge early Tuesday, ship tracking website MarineTraffic showed.
The logs show the ship was sailing from Baltimore to Colombo, Sri Lanka.
The BBC reported that Synergy Marine Group had confirmed that its vessel was involved.