A search operation is continuing for at least six people thrown into a river in the US city of Baltimore after this huge container ship collided with a bridge, sending it crashing into the water. Maintenance workers were working on the bridge when it happened in the early hours of the morning. Two people were pulled from the freezing waters. Rescuers are now using Sonar to search the wreckage for the missing. In the minutes before the crash, it's thought that the container ship had lost power. Well, in a moment, Rebecca Morrell will take a step-by-step look at exactly what happened. But first, we have correspondence on both sides of the bridge in Baltimore. Gary O'Donaghue is on the east side, and Sumi Somaskanda is on the west. Sumi, first of all, the progress so far of the rescuers?

Sophie, there is a massive search and rescue operation underway. It's going to continue into the night. As you mentioned, they are using sonar detectors to try to more precisely locate where those missing workers might be. We have also learned that FBI divers have joined those rescue and recovery boats, and that they are using robotic cameras to also try to locate where those missing workers might be. Now, a law enforcement source who told us that information was not able to say whether there was one or more divers in the water at this moment, and that is because of the jagged debris. You have to remember that massive chunks of metal from this bridge fell right into the river, really complicating efforts. And that indeed corresponds with what we heard from Maryland Senator Ben Cardon, who we spoke to just about half an hour ago, who told us one of the problems in the search and rescue mission at the moment is precisely that they really had to maintain a surface search because getting deeper into the water has not been possible because of that debris. They're going to continue searching, Sophie, but several factors working against them at this hour, that debris, the wreckage from the bridge, but of course, also weather.

It is very cold, and as the evening moves on, it will, of only get colder, and the water conditions. The water, very choppy, murky, making this effort all the more difficult.

Gary O'Donahue, the images are extraordinary. A lot of people are very surprised at just how fast this bridge collapsed.

Yeah, Sophie, I've been here since before day break, and we were talking to people down by the water's edge, out walking their dogs, people who live very close to the river here, and they were in total shock at what had happened. This is a massive a massive landmark in their lives, something they use on a daily basis, and it had simply disappeared overnight. Some heard the crash. They said they felt their houses shake as the bridge went down. Now, of course, the bridge was named after Frances Scott Key, the author who inspired the words for the Star-Spangled Banner. One woman said to me, It was a symbol, she said, a symbol of America. This is how the day's events unfolded. It's just before 1:30 in the morning, and the 985-foot DALL-E container ship is heading for the open sea. Port on traffic on the Key Bridge. There's a ship approaching and just lost their steering. Until you get that under control, we got to stop all traffic. The ship had lost power and was veering straight into one of the bridge's pylons. C13, this is the The whole bridge just fell down. Start as everybody.

The whole bridge just collapsed. Eight members of a construction team working on the bridge filling potholes were thrown into the ice-cold water. Two were rescued, one of whom is critically ill in hospital. Incredibly, the crew on the ship, realizing something was wrong, managed to issue a mayday before hitting the bridge. That allowed the authorities on land to prevent more vehicles from going onto the bridge, unquestionably saving lives. Throughout the night and day, boats, helicopters, and divers have been scouring the water for survivors. The authority is vowing to continue the search for as long as it takes. This is very much still a search and rescue mission. We are still actively looking for survivors. We know, and that's the pledge we've made to these families, and this is still very much an active search and rescue mission. Local residents were woken in their beds by the sound of the crash, devastated by the sheer scale of the destruction. My husband does that road work. I don't know what could have been his friend scared.

It's scary, and it fucks.

It's the Key Bridge.

It's just a move of America.

It's very sad. It's very sad.

Local investigators have made it clear they believe this was an accident and have ruled out any of terrorist incident.

I've directed my team to move heaven and earth to reopen the port and rebuild the bridge as soon as humanly possible. We're going to work hand in hand with the support of Maryland to support Maryland and whatever they ask for.

For now, the Port of Baltimore, one of the 10 largest in the United States, remains closed, and the loss of the bridge severs a key artery for large vehicles on America's Eastern Seaboard. The impact of this accident will be felt for months to come. Gary O'Donagh, BBC News, Baltimore.

So why did the ship go off course, and why did this huge bridge come crashing down so easily? Rebecca Morrell is here. She's been looking at the sequence of events.

Thanks, Sophie. Well, what do we know about what happened in Baltimore? Data analyzed by BBC Verify shows the ship set off from the port at a quarter to one in the morning local time. The Dali is a 300-metre long container ship, and on board were two pilots, mariners from the local ports with a specialist knowledge about how to guide the vessel out to sea. Now, it was heading towards the central section of the bridge, which is wide enough and tall enough for the ship to pass through, but it veered off course and crashed into this column here. So what happened? Well, in this video, you can see the ship is nearing the bridge, but at 1:24 AM, the lights go off suddenly, suggesting a major technical fault. A minute later, they come back on again. Now, three minutes after this, you start to see the black smoke billowing from the ship, and Over here where you can see this circle, are the construction vehicles. Workers there were fixing potholes on the road. Then moments later, a mayday was issued and you could see the moments of impact and the bridge collapsed. If you lose power while maneuvering, there's a possibility you could lose control of the steering of the vessel momentarily.

There are regulations as to how quickly you have to be able to regain that control. If that does happen, obviously, it's such a confined area, those seconds or minutes can make the difference between having a collision or not.

Why did such a large part of the bridge collapse? Well, the bridge opened in 1977, and at its center is a continuous section spanning 366 meters long. There are four supporting columns underneath it. Now, bridges are tested for impacts, but we don't know if calculations would have included a vessel of this size. Ships are a lot bigger now than they were in the '70s. The head-on collision took out an entire load-bearing support. Now, protective reinforcements can be added to these, but it's not clear if they were here.

You have a long element that's supported by four supports ports, and you're simply removing one of them.

I could not imagine another situation where a bridge like that would resist such a massive force.

A major search effort is still going on, using sonar to try and locate any vehicles that fell into the 15 meters of water below. And an investigation into the tragedy has already begun.

Sophie.rebecca, thank you.