The collapse of Baltimore's Francis Scott Key Bridge after it was struck by a cargo ship has left the city and Maryland heartbroken, Gov. Wes Moore said, as a the search for survivors continued.

The collapse sent cars and construction workers on the bridge plunging into the frigid waters below.

"Our state is in shock," Gov. Moore said at a mid-morning update. "This has been a long night."

He said investigators were quickly gathering details on the incident but stressed that it appeared to be an accident and there was no indication of a link to terrorism.

The ship's crew reported losing power before the incident, Moore said.

"I recognize that many of us are hurting right now. I recognize that many of us are scared right now," Moore said.

"All of our hearts are broken," he said.

Baltimore Mayor Brandon Scott earlier called the collapse "an unspeakable tragedy."

State Transportation Secretary Paul Wiedefeld said six people remained unaccounted for, while two others had been found: one in the hospital, one not.

Baltimore Fire Chief James Wallace said about 50 divers were in the water for the search.

They have located three passenger vehicles in the water and a cement truck. He says there is one other possible vehicle that was located.

In an interview on CNN, Wallace said the wreckage in the water presented challenges and crews were worried about a secondary collapse.

Some 30,000 people use the bridge daily, the governor said. He noted that as soon as the mayday call went off, crews stopped cars from traveling onto the bridge, saving lives.

The National Transportation Safety Board will join the investigation and planned an update later on Tuesday.