(CNN) — Federal investigators unveiled new details about what occurred in the minutes before a hulking cargo ship lost power and slammed into Baltimore’s Francis Scott Key Bridge, including the pilot’s urgent call for assistance and authorities’ efforts to clear people off the bridge.

The first sign of distress came just under three minutes before the crash when the cargo ship’s pilot called over the radio requesting any tugboats in the area to respond to the vessel, National Transportation Safety Board Chair Jennifer Homendy said in a news conference Wednesday.

Within a minute, police officers on either end of the bridge were ordered to stop traffic crossing the bridge, said Marcel Muise, the NTSB investigator in charge of the collapse inquiry – an action several officials have credited with saving lives.

Investigators had their first full day at the scene Wednesday and witnessed the “utter devastation” of the mangled bridge, parts of which are still draped over the ship’s bow, Homendy said.

“When I look at something like that, I am thinking not about the container ships that are coming through, not about traffic getting back up and running on the bridge. I’m thinking about the families who’ve lost loved ones,” Homendy said.

Six construction workers who were on the bridge are now presumed dead, and two of their bodies were found trapped in a submerged truck on Wednesday, police said. Search efforts for the remaining four have been paused until salvage crews can clear heavy underwater debris that is believed to be encasing their remains.

NTSB crews used the ship’s voyage data recorder, or VDR, to piece together a rough timeline of events leading up to the collision. But it will take months for them to gather the piles of physical evidence, maintenance records, ship data and witness interviews required to deliver a full report, Homendy said.

Here’s the NTSB’s timeline in hours, minutes and seconds:

• Approximately 12:39 a.m.: The ship departed from Seagirt Marine Terminal.
• By 1:07:00 a.m.: The ship had entered the Fort McHenry Channel.
• 01:24:59 a.m.: Numerous audible alarms were recorded on the ship’s bridge audio. At about the same time, the VDR stopped recording ship system data but was able to continue recording audio using a different power source.
• 01:26:02 a.m.: The VDR resumed recording ship system data. During this time, steering commands and orders regarding the rudder were captured on audio.
• 01:26:39 a.m.: The ship’s pilot made a general very high frequency (VHF) radio call for tugboats in the vicinity to assist the vessel. Around this time, the pilot association dispatcher contacted the Maryland Transportation Authority duty officer regarding the blackout, according to transit authority data.
• Around 01:27:04 a.m.: The pilot ordered that the ship’s port anchor be dropped and issued additional steering commands.
• Around 01:27:25 a.m.: The pilot issued a radio call over the VHF radio, reporting that the vessel had lost all power and was approaching the bridge. Around this time, the transit authority duty officer radioed two of its units — one on each side of the bridge — that were already on scene and ordered them to close traffic on the bridge. All lanes were then shut down.
• Around 01:29 a.m.: The ship’s speed over ground was recorded at just under 8 miles per hour. From this moment until approximately 1:29:33, the VDR audio recorded sounds consistent with the collision with the bridge. Additionally, MDTA dash cameras show the bridge lights extinguishing.
• 01:29:39 a.m.: The pilot radioed the US Coast Guard to report the bridge was down.

There were 23 people onboard the cargo ship when it collided with one of the bridge’s columns, including 21 crew members and two pilots, who are tasked with getting the ship out of port. All crew members were safe, the Maritime and Port Authority of Singapore said previously.

There were no issues reported with the ship prior to its arrival in Baltimore, officials said Wednesday. “We were informed that they were going to conduct routine engine maintenance on it while it was in port. And that’s the only thing we were informed about the vessel in that regard,” Coast Guard Rear Admiral Shannon Gilreath said at the news conference.

As the investigation continues, NTSB and debris salvage crews face challenging and dangerous conditions, including cold and rainy weather, slick surfaces and unstable pieces of wreckage, NTSB and fire officials said.

“Naturally, we’re still very cognizant of the fact that there are hazardous materials aboard the vessel itself,” Baltimore City Fire Chief James Wallace said Wednesday.

Among the ship’s cargo, a senior NTSB hazmat investigator has identified 56 containers of hazardous material – 764 tons – mostly corrosives and flammables, as well as some lithium-ion batteries, the agency said.

Earlier, Coast Guard Vice Admiral Peter Gautier said there is no hazmat threat to the public. Of the ship’s 4,700 cargo containers, only two are missing overboard and neither contains hazardous materials, he said.

The collapse has left massive steel structures and cement debris in the Patapsco River, as well as several vehicles that may contain victim’s remains, police said. Divers stopped their search efforts Wednesday until salvage crews can remove enough of the materials for it to become safe to reenter the water, Maryland State Police’s Col. Roland L. Butler Jr. said.

Unlike most bridges built today, the Key Bridge – completed in 1977 – is “fracture critical,” Homendy explained. “What that means is if a member fails that would likely cause a portion of, or the entire bridge, to collapse. There’s no redundancy.”

The destruction of the bridge and the resulting closure of the port will have major repercussions on the city’s economy and the country’s supply chain, Transportation Secretary Pete Buttigieg said Wednesday. The Baltimore port is the largest in the US for autos and light trucks, handling a record 850,000 vehicles last year.

Recovery operations temporarily halted

Eight construction workers were believed to be mending potholes on the bridge when it fell, according to officials. Two survived but crews searched the frigid waters throughout the day Wednesday for the remaining six, who were all presumed dead.

Two of the workers, a 35-year-old and a 26-year-old, were found trapped underwater in a red truck Wednesday. They were identified by authorities as Alejandro Hernandez Fuentes, from Mexico, and Dorlian Ronial Castillo Cabrera, from Guatemala. Both lived in the Baltimore area.

Search efforts to recover the bodies of the remaining four men were paused later Wednesday until search conditions could be made safer for divers, said Butler, the state police superintendent.

Based on sonar scans, officials believe that the vehicles are “encased in the superstructure and concrete” of the bridge, Butler said.

“The sonar simply said they cannot get to that area because it was fully encased in the superstructure,” he said. “Once that salvage effort takes place and that superstructure is removed, those same divers are going to go back out there and bring those people closure,” he added.

Among the other victims was Miguel Luna, a father of three, and Maynor Suazo, a father and entrepreneur.

The Ministry of Foreign Affairs in Guatemala said another Guatemalan was also unaccounted for: a 35-year-old from Camotán, Chiquimula. Another of the missing is also a Mexican national, Mexican president Andrés Manuel López Obrador said.

Bridge has been struck before – and survived

Tuesday’s catastrophe was not the first time a vessel has slammed into the Key Bridge. Four decades earlier, another container ship that also lost power hit the same bridge — and it stood strong.

The dramatic difference in outcomes between the two accidents is an example of the dangers caused by the massive increase in shipping vessel size in the intervening decades.

A CNN review of public records and interviews with about a dozen bridge and shipping experts show that hundreds of bridges over US waterways were built decades ago when container ships were a fraction of the size and weight they are today. Bridges of the era when the Key Bridge was built weren’t designed to protect against collisions with ships as big as the Dali, the vessel that caused the Baltimore to topple.

The Dali has a capacity of about 10,000 twenty-foot equivalent units of cargo – compared to the approximate cap of about 2,500 twenty-foot equivalent units that could be carried by container ships in the 1970s, CNN has reported.

Some experts said that this week’s disaster should inspire engineers to reevaluate whether America’s aging infrastructure can withstand impacts from the gigantic ships that traverse our waterways today.

“It’s absolutely a wake-up call,” said Rick Geddes, a professor and director of Cornell University’s Program in Infrastructure Policy. “The people who were building the Francis Scott Key Bridge never really contemplated ships of this size. It wasn’t their fault — they just didn’t have a crystal ball.”

Ship and management company involved in previous incidents

As officials worked to investigate the collapse Tuesday, questions emerged over previous issues with the ship and its management company.

The ship was briefly held at the Port of San Antonio in Chile on June 27, 2023, when an inspector found that the pressure gauges for the vessel’s heating system were “unreadable,” a spokesperson for the Chilean Navy said.

Ships managed by the Synergy Marine Group have been involved in at least three deadly incidents since 2018 in Australia, Singapore and the Philippines, according to officials in those countries.

In 2018, a member onboard of a vessel managed by Synergy in Australia was killed in an accident involving the ship’s personnel elevator, according to a report from the Australian Transportation Safety Bureau.

In 2019, an officer on a Synergy-registered vessel in Singapore was reported missing after “likely (falling) overboard while performing inspection or cleaning jobs,” according to a report by the Transport Safety Investigation Bureau of Singapore’s Ministry of Transport.

In 2023, at least one sailor was killed when a Synergy Marine-managed tanker collided with a dredging ship in the Philippines, causing it to capsize, according to an incident report from the Philippines Coast Guard.

Synergy Marine Group is focused on the Baltimore incident, spokesperson Charlie Ridgeway told CNN in a phone call Wednesday. “It would be inappropriate to discuss any previous incidents at this time.”

CNN’s Tori B. Powell, Sarah Engel, Elise Hammond, Curt Devine, Casey Tolan, Isabelle Chapman, Daniel Medina, Sahar Akbarzai, Melissa Alonso, Hande Atay Alam and Lucas Lilieholm contributed to this report.