The crew of a massive container ship that crashed into the Francis Scott Key bridge in Baltimore early Tuesday warned of power issues before the collision, which caused the bridge to collapse into the frigid Patapsco River, officials said.

At least six people believed to be part of a construction crew that was repairing potholes on the bridge remain unaccounted for, Maryland Transportation Secretary Paul J. Wiedefeld said at a news conference Tuesday.

Two people have been pulled from the Patapsco River, Baltimore City Fire Department Chief James Wallace said. One of them was not injured and the other was taken to a local trauma center in “very serious condition,” he said.

Maryland Gov. Wes Moore said the warning from the ship’s crew likely saved lives.

“I’m thankful for the folks who, once the warning came up, and once notification came up that there was a mayday, who literally by being able to stop cars from coming over the bridge, these people are heroes,” Moore said.

Several vehicles are believed to have fallen into the water, including one as large as a tractor-trailer, Kevin Cartwright, director of communications of the Baltimore City Fire Department said early Tuesday.

“We have an unspeakable tragedy,” Baltimore Mayor Brandon Scott told CNN.

“There were individuals working on the bridge at the time. There are cars in the water – our fire department has confirmed that as they lead this search-and-rescue mission through sonar. That is where our focus is – it’s about those souls, those people we’re trying to find and get out of this water,” Scott said.

Construction workers contracted with the state transportation agency were doing repair work on the bridge at the time of the collapse, Maryland Transportation Secretary Paul Wiedefeld said Tuesday morning. He did not specify how many workers were there.

The ship, which hit the bridge just before 1:30 a.m., was the DALI, a Singaporean-flagged container vessel, public affairs officer for the US Coast Guard’s 5th District, Kimberly Reaves, said. It is about 984-feet long, according to MarineTraffic data.

Lights on the ship flickered and a dark plume of smoke could be seen billowing from it before it veered towards a bridge pillar shortly before impact, CNN analysis of data from the MarineTraffic ship-tracking website shows.

No crew members on the ship were injured, ship management company Synergy Group said a statement.

Multiple agencies are investigating the crash, though there is currently “absolutely no indication that it was intentional,” Baltimore Police Commissioner Richard Worley said Tuesday morning.

The 1.6-mile, four-lane bridge extends over the Patapsco River and serves as the outermost crossing of the Baltimore harbor and an essential link of I-695, or the Baltimore Beltway.

Here are the latest developments:

• Multiple air and marine units have been deployed for the search, and personnel from the local fire and police departments, as well as the Coast Guard and Baltimore FBI are on scene to assist, officials said Tuesday.
• Maryland Gov. Wes Moore declared a state of emergency Tuesday morning.
• The White house is closely monitoring the situation, an official told CNN.
• US Transportation Secretary Pete Buttigieg said he is offering federal support in response to the collapse.
• Danish Shipping company Maersk told CNN it chartered the ship, which was carrying Maersk customer cargo. The company said no Maersk personnel were onboard the vessel, which is operated by charter vessel company Synergy Group.
• The ship has been inspected 27 times since its was built in 2015 and has had two “deficiencies,” including one for structural damage, according to records from the Electronic Quality Shipping Information System.

Rescue effort underway

Dive operations in the search and rescue effort have begun, Baltimore County Executive Johnny Olszewski told CNN.

“The conditions are difficult,” Olszewski said. “We’re talking about a deep channel port. It’s 40, 50 feet of water, strong currents. The weather is windy, the water is cold. And so we certainly worry about those who are in the water, not to mention the fall from the bridge.”

Teams were working “methodically and safely” to ensure “everyone operating here on the scene is safe and that we’re able to make progress without causing adverse harm to anyone else,” Cartwright said.

High tide could present a challenge for rescuers looking for people, Wallace, the fire chief, said.

Officials will rely on the expert divers assisting with the search “to tell us when they believe we’ve reached that non-survivability point,” Wallace said.

All traffic is being detoured away from the bridge, said the Maryland Transportation Authority.

Video of the collision shows a towering boat headed directly toward one of the bridge’s support columns before colliding with it, sending a massive stretch of the bridge crashing into the water below in mere seconds. The impact immediately triggered the collapse of adjacent portions of the bridge.

The crash sent large plumes of smoke and fire into the air and part of the bridge appeared to collapse over the front of the boat, video footage showed. Dark smoke continued to rise into the air for several minutes.

The bridge’s namesake, Francis Scott Key, is believed to have sat near the site of the bridge as he witnessed the bombardment of Fort McHenry in 1814, inspiring him to write the words of the “Star Spangled Banner.”

This is a developing story and will be updated.

CNN’s Holly Yan, Holmes Lybrand, Sahar Akbarzai, Andy Rose and Chris Boyette contributed to this report.