BALTIMORE — Just hours before the Tuesday morning commute was to get underway, a massive cargo ship leaving Baltimore harbor lost propulsion and crashed into a support column of Baltimore's 1.6-mile long Francis Scott Key Bridge, causing a partial collapse and sending vehicles and people into the water, officials said.

The transportation disaster unfolded about 1:35 a.m., triggering a major emergency response from Baltimore police, firefighters and the U.S. Coast Guard as authorities estimated that up to 20 vehicles went into the water along with several workers who were part of a maintenance team fixing potholes on the span, officials said.

"I can tell you, our sonar has detected the presence of vehicles submerged in the water," Baltimore City Fire Chief James Wallace said at a news conference early Tuesday.

Hear the latest from ABC Audio's Derricke Dennis on the scene:

Wallace said dive teams were probing the water for survivors. He said the challenges to rescue crews included darkness, maneuvering through a massive and dangerous debris field and the swift current that runs through the channel. Asked how long someone could survive in the frigid water, Wallace said, "We're going to rely on the experts, which is our dive masters that are here, our dive team, to tell us when they believe we've reached that non-survivability point."

Maryland Gov. Wes Moore said at a news conference Tuesday morning that rescue crews are doing all they can to save lives.

"To the victims of this tragedy and their loved ones, all our hearts are broken," Moore said. "In the face of heartbreak, we come together, we embrace each other."

Emergency officials said eight people were initially unaccounted for and two were rescued. At least six people were still missing. One of those rescued was in "very serious" condition. The University of Maryland Medical Center said it treated one patient rescued from the bridge collapse and that person had been discharged from the hospital.

Moore described the crash as "unprecedented."

"To hear the words that the Key Bridge has collapsed, it's shocking and heartbreaking," Moore said.

Officials said the container ship was moving at a speed of 8 knots, or about 9 mph, when it struck the bridge. They said the disaster could have been much worse had authorities not stopped cars from going onto the bridge.

According to a Coast Guard memo obtained by ABC News, a harbor pilot and an assistant aboard the cargo ship reported the power issues that prompted multiple alarms on the bridge of the vessel and loss of propulsion.

All workers on the container ship were accounted for, according to the memo.

The cargo ship pilot is expected to undergo post-accident drug and alcohol testing.

Moore said there was no credible evidence that the crash involved terrorism. He said it appeared to be a tragic accident.

The FBI, which arrived at the scene an hour after the incident, confirmed that no link to terrorism was involved, according to Bill DelBagno, special agent in charged of the FBI's Baltimore field office.

Moore said there were no structural issues with the bridge, saying it was "fully up to code."

It still remains unclear what caused the loaded 984-foot container ship, Dali, a Singapore-flagged vessel, to crash into the bridge about a half hour after it began its intended journey out of the second-largest seaport in the Mid-Atlantic via the Patapsco River, which the four-lane Francis Scott Key Bridge, named after the amateur poet who wrote the Star-Spangled Banner in 1814, crosses.

Danish shipping company Maersk said it chartered the Dali cargo ship, a spokesperson for the company told ABC News in a statement.

“We are horrified by what has happened in Baltimore, and our thoughts are with all of those affected. We can confirm that the container vessel “DALI”, operated by charter vessel company Synergy Group, is time chartered by Maersk and is carrying Maersk customers’ cargo. No Maersk crew and personnel were onboard the vessel. We are closely following the investigations conducted by authorities and Synergy, and we will do our utmost to keep our customers informed," the Maersk spokesperson said.

Dramatic security video captured the vessel striking one of the main support columns holding up the center cantilevered section of the bridge, causing the span to break apart in several sections and sending twisted metal into the water onto the bow of the Dali as black smoke began to pour from the vessel.

Multiple vehicles plunged from the bridge at the time of the collapse, the Baltimore City Fire Department said.

Just minutes before the crash, the video showed traffic flowing on the bridge, but the traffic almost disappeared before impact.

Had the crash occurred a few hours later at the height of the morning commute the bridge would have likely been packed with commuters. The bridge is part of the heavily traveled Interstate 695 linking Baltimore to Washington, D.C. An estimated 11.5 million vehicles cross the bridge annually, or about 30,000 per day, according to the Maryland Department of Transportation.

The bridge, which opened on March 23, 1977, had just celebrated its 47th anniversary.

The crash shut down the seaport, which serves more than 50 ocean carrier companies whose vessels make about 1,800 annual visits to the port annually, according to state officials.

ABC News' Victoria Arancio, Alex Grainger, Sam Sweeney and Felicia Alvarez contributed to this story.

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