(Adds Biden visiting Baltimore on Friday in third paragraph)

By Lisa Shumaker

April 1 (Reuters) - Work is underway to begin clearing

the wreckage of Baltimore's Francis Scott Key Bridge a week

after a cargo ship crashed into it, sending the span crashing

into the harbor and killing six construction workers.

Replacing the bridge will likely take years, but authorities

are working on opening a temporary alternate channel that will

be 11 feet (3.4 meters) deep to allow some vessels through.

President Joe Biden will visit Baltimore on Friday.

WHAT IS THE DEATH TOLL?

The six victims of the bridge collapse were all immigrants from

Mexico and Central America who were fixing potholes on the

bridge on March 26.

Divers recovered the bodies of two men the following day.

They were identified as Alejandro Hernandez Fuentes, 35, of

Baltimore, originally from Mexico, and Dorlian Castillo, 26, of

nearby Dundalk, originally from Guatemala.

Authorities have not yet recovered the bodies of the other

four men in the 50-foot-deep (15 m) waters surrounding the

twisted ruins due to treacherous conditions. They are Maynor

Suazo from Honduras; Jose Lopez from Guatemala; Miguel Luna from

El Salvador; and another whose name has not been released. Two

other workers were rescued.

Authorities saved lives by stopping vehicles from using the

bridge after the ship sent out a mayday call.

The ship also dropped its anchors to slow down, buying time

to clear the bridge.

WHEN DID THE BALTIMORE BRIDGE COLLAPSE?

Shortly after 1 a.m. EDT (0500 GMT) on March 26, a container

ship named the Dali was traveling down the Patapsco River on its

way to Sri Lanka. At 1:24 a.m., it suffered a total power

failure and all its lights went out.

Three minutes later, at 1:27 a.m., the container ship struck

a pylon of the bridge, crumpling almost the entire structure

into the water.

Less than a minute before impact, a first responder on

emergency radio responded to the crew's mayday call by sending

officers to halt traffic onto the bridge.

Without their fast work, the scale of the disaster may have

been far greater, even during the early morning hours when

vehicular traffic is relatively light.

The disaster may be the worst U.S. bridge collapse since

2007, when a design error caused the I-35W bridge in Minneapolis

to plunge into the Mississippi River, killing 13 people.

WHY DID THE BRIDGE COLLAPSE?

Bridges such as the one in Baltimore are classified as "fracture

critical" by the federal government - meaning that if one

portion of the bridge collapses, the rest of the structure

falls. There are more than 16,800 such spans in the U.S.,

according to the Federal Highway Administration.

The head of the National Transportation Safety Board said

the bridge lacked structural engineering redundancies common to

newer spans, making it more vulnerable to catastrophic collapse.

The Key Bridge opened in 1977 - three years before a similar

vessel collision of the Sunshine Skyway Bridge in Tampa Bay,

Florida, killed 35 people, and prompted bridge designers to

implement better protections for foundation piers.

WHO WILL PAY FOR THE DAMAGE AND HOW MUCH WILL THE BRIDGE

COST?

President Joe Biden will visit Baltimore on Friday and has said

he wanted the federal government to pay to rebuild the bridge.

The Transportation Department on March 28 awarded $60 million in

"quick release" emergency relief funds to aid in clearing debris

and begin the process of rebuilding. To replace the bridge,

Congress would need to approve funding. After the bridge

collapse in 2007 in Minnesota, Congress allocated $250 million.

Initial estimates put the cost of rebuilding the bridge at

$600 million, according to economic analysis company IMPLAN.

Federal officials have told Maryland lawmakers the final

cost of rebuilding the bridge could soar to at least $2 billion,

Roll Call reported, citing a source familiar with the

discussions.

Insurers could face billions of dollars in claims, analysts

said, with one putting the cost at as much as $4 billion, which

would make the tragedy a record shipping insurance loss.

HOW LONG WILL IT TAKE TO REBUILD THE BRIDGE?

Rebuilding could be a lengthy process and will depend on whether

any of the remaining structure can be salvaged. It took five

years to construct the original bridge from 1972-1977.

The closure of the port for just one month would cost

Maryland $28 million in lost business, according to IMPLAN.

Authorities are preparing to establish a temporary channel

on the northeast side of the main shipping lane to allow

essential vessels through.

WHAT SHIP HIT THE BALTIMORE BRIDGE?

The Dali was leaving Baltimore en route to Colombo, Sri

Lanka, with 21 crew and two pilots on board.

The ship measures 948 feet (289 m) - as long as three

football fields. It was stacked high with containers but capable

of carrying twice as much cargo. Safety investigators recovered

the ship's black box, which can give them the vessel's position,

speed, heading, radar, bridge audio, and radio communications as

well as alarms.

The same ship was involved in an incident in the port of

Antwerp, Belgium, in 2016, when it hit a quay as it tried to

exit the North Sea container terminal.

A later inspection in June 2023 carried out in San Antonio,

Chile, found the vessel had "propulsion and auxiliary machinery"

deficiencies, according to data on the public Equasis website,

which provides information on ships.

The registered owner of the Singapore-flagged ship is Grace

Ocean Pte Ltd, LSEG data show. Synergy Marine Group managed the

ship, and Maersk chartered the vessel.

WHAT DO WE KNOW ABOUT THE BRIDGE THAT COLLAPSED?

The Francis Scott Key Bridge was one of three ways to cross

the Baltimore Harbor and handled 31,000 cars per day or 11.3

million vehicles a year.

The steel structure was four lanes wide and rose 185 feet

(56 m) above the river.

It opened in 1977 and crosses the Patapsco River, where U.S.

national anthem author Francis Scott Key wrote the "Star

Spangled Banner" in 1814 after witnessing the British defeat at

the Battle of Baltimore and the British bombing of Fort

McHenry.

HOW WILL THE BRIDGE COLLAPSE IMPACT THE BALTIMORE PORT?

Traffic was suspended at the port, the 17th largest in the

country, and the jobs of 15,000 people are on hold.

The flow of containers to Baltimore can likely be redistributed

to bigger ports. However, there could be major disruptions in

shipping cars, coal and sugar.

It is the busiest U.S. port for car shipments, handling at

least 750,000 vehicles in 2023, according to data from the

Maryland Port Administration.

In 2023, the port was the second busiest for coal exports.

It is also the largest U.S. port by volume for handling farm

and construction machinery, as well as agricultural products

such as sugar and salt.

(Writing by Lisa Shumaker; Editing by Daniel Wallis and Bill

Berkrot)