A total power and propulsion failure like the one the cargo ship Dali experienced before it struck and toppled the Francis Scott Key Bridge is an extremely rare occurrence, one possibly caused by either faulty equipment or contaminated fuel, maritime engineers and experts said.

After the power failed, the ship coasted in the darkness before the generators and engine appeared to kick back on and its lights flickered before going dark again. The ship’s pilot dropped anchor and tried to steer away, but without power to the propeller, impact was almost certain.

Just moments later, the Dali slammed into the bridge, killing six construction workers who were filling potholes. The bridge’s collapse has shut down shipping into Baltimore for the foreseeable future.

The National Transportation Safety Board is still in the nascent stages of its investigation into the crash, but officials confirmed at a press briefing Wednesday evening they would look into whether watered down or contaminated fuel played a role in the ship’s total systems loss.

“As part of any investigation by the NTSB we look at the fuel system. We collect a sample of the fuel,” NTSB Chairwoman Jennifer Homendy said. “But we still have to do that.”

The Wall Street Journal first reported the NTSB was examining contaminated fuel as part of its inquiry.

Ships suffer from mechanical and electrical issues from time to time, but it’s highly unusual for all systems to fail at once, experts said, meaning a larger malfunction is likely to blame.

“I have never seen where a main propulsion engine and a generator fail at the same time,” said Henry Lipian, a forensic crash reconstructionist and retired U.S. Coast Guard lieutenant in Ohio.

The exact cause of the catastrophic failures aboard the Dali, a ship larger than any downtown Baltimore skyscraper, likely won’t be known until the NTSB progresses further in its investigation. A preliminary report is expected sometime in the next month, and the full inquiry could last two years.

It’s possible, even likely, that the Dali experienced a one-off mechanical failure that just happened to occur at the worst possible time. Anytime a commercial vessel leaves port in American waters, Coast Guard regulations require a ship’s crew complete what’s known as a “getting underway checklist,” which includes an inspection of the propulsion system, or engine, and the emergency generator.

NTSB investigators will review the Dali’s logs in addition to its voyage data recorder, which captures some systems information and records audio on the vessel’s bridge.

There is a record of previous safety and propulsion issues with the Dali, but there’s no indication they are related to Tuesday’s catastrophe.

In 2023, authorities in San Antonio, Chile, cited the Dali for a propulsion issue, but the ship’s crew made the needed repairs within a day and the vessel was allowed to set sail, according to inspection records. The Dali was also involved in a smaller allision in Antwerp, Belgium, in 2016, records show.

A ship passing all of its prevoyage safety measures does not mean things will continue to work for the immediate future, said Michael Buckley III, a chief engineer with Maersk Line Limited, a subsidiary of the Danish shipping giant.

“Mechanical failure is something you plan preventative maintenance for,” Buckley said. “But you’re not guaranteed that piece of equipment isn’t going to fail at any given point. There’s no crystal ball.”

Large cargo vessels usually run on diesel fuel, which powers their engines and the generators that provide electricity to the vessel. In addition to its usual generators, each ship is required to have a backup generator, which is supposed to automatically kick on when the others go out, said Tim McCoy, a professor of marine engineering at the University of Michigan. For it all to fail at once, and to fail where it did, is extremely rare and unlucky, he said.

“There’s over 55,000 merchant ships in the world today, over 5,000 container ships, and how many bridges get struck? How many of those ships are going under bridges every day?” McCoy said. “I can’t do the math on that off the top of my head, but I think you can agree it’s a rare event.”

There is speculation that perhaps the ship took on either contaminated or so-called “dirty fuel,” or the wrong grade of diesel. Buckley has sailed the seas for 35 years, the last 20 as a chief engineer, and said contaminated fuel, usually high in silicones, is becoming more common.

Ships like the Dali have on-board fuel filtration systems, meant to clean out impurities and to remove excess water, but they’re not perfect. Fuel filters leading to engine components, like fuel pumps, can get clogged. If a pump fails and pressure gets low, so does the engine.

“I would basically wager they had a fuel system problem,” Buckley said.

If there was a problem with the fuel, Lipian said, it would make sense the ship could run for awhile before bad fuel entered the engine. Lipian said he has a myriad of questions, including whether the ship filled up in the port and where the fuel came from.

Lipian said the black smoke that billowed from the ship after the total blackout of equipment indicated to him that perhaps the crew had been able to restart the engine and get the generator — or the backup generator — on.

“It is increasingly rare that you see black smoke coming out of a diesel engine,” he said. “They belched a lot of black smoke.”

The smoke he believes shows that the crew did not get the engine running efficiently, a sign perhaps of the fuel problem.

Lipian wants to know whether the ship left the terminal before “everything got up to correct operational temperatures.”

But another expert, Michael Kucharski, a former investigator for the Coast Guard and the NTSB who has extensive experience on large vessels, said that, while possible, he is skeptical of the theory that contaminated fuel was the problem.

“My best guess is that they had some kind of main engine problem. They lost the main power to the main engine … so that doesn’t give the power, enough energy, for the generator and the propeller. The generator may not be providing electricity to the propeller.”

Looking at the video of the crash, he said, it appears that the lights flicker back on, suggesting that the emergency generators came back on within a minute. That should have provided some energy to restart the propeller.

Some components that interface with a ship’s engine are powered by its generators, McCoy said, and an electrical outage will lead to engine failure. An electrical shortage or a fire could overload the ship’s grid, causing other problems.

“Things like fuel and cooling pumps and things like that, are typically electrically powered,” he said. “If you lose all electric power on the ship then generally the propulsion system is going to stop working.”

Lipian’s only other theory is that the ship’s failure was caused by human error — a crew member mistakenly pressed something on a control panel that shut down both the generator and the engine at the same time.

“I think that is a remote possibility,” he said.

Whatever the causes of the shutdown, there was only three minutes for the crew to react. In those final moments, the pilot gave an order to turn the rudder left, sent a mayday call, and had the anchor dropped. Once the crew dropped the anchor, it would have landed at the bottom of the channel in under 10 seconds, Kucharski said.

But, by then, the ship was about two minutes away from hitting the bridge and there was no way out.

A final accounting of the Dali’s voyage will come in years, and might give a picture that is far different than what experts see less than a week from the accident.

“What they think happened and what actually happened are usually quite different,” said Kucharski, who said he has interviewed dozens of individuals for crash investigations. Little pieces of information — a sight, smell or sound — could be critical clues to the investigation, he said. “Sometimes, the things that you think are inconsequential turn out to be big.”

Banner data reporter Greg Morton contributed to this article.