Pennsylvania authorities were working to remove seven barges that were trapped at a dam downriver from Pittsburgh on Tuesday after 26 barges broke loose and drifted down the Ohio River last week.

On Monday, the company that owns the barges announced it had located a vessel previously declared missing using sonar technology.

The barges drifted freely down the river before some of them came to rest on two locks and dams on the Ohio River downstream from Pittsburgh.

Pittsburgh is a historically industrial Pennsylvania city known for its iconic bridges that swoop over rivers in and around the city. The Ohio River's three locks and dams handle an average of 50 million tons of activity every year, according to the Pittsburgh Ports Commission, making them integral to the flow of shipping in the area.

Barges are long vessels used to transport large amounts of cargo – goods and materials ranging from coal and steel to corn and soybeans. They play an important role in the ocean shipping industry, which transports around 80% of all goods worldwide, according to Statista. It's unclear how the barges became loose.

Here's what to know about the continuing recovery effort.

When did the barges break loose?

Reports came in of 26 barges that broke loose on the Ohio River on Friday night at around 11:25 p.m., according to the Pittsburgh fire and police departments.

The Army Corps of Engineers said in a Facebook post that one of the barges that came to rest at Emsworth Locks and Dam was removed on Saturday morning. Four other barges that drifted past both dams into Moon Township were retrieved by tugboats.

Carol Vernon, a spokesperson for the Pittsburgh District Army Corps of Engineers, told USA TODAY it was "too early" in the recovery process to open an investigation into the cause of the incident.

"The concentration now is primarily on making sure that the barges can be removed safely and to get navigation on the rivers back open and operating," she said.

How many barges have been removed?

Personnel with the U.S. Army Corps of Engineers (USACE), the U.S. Coast Guard, and Campbell Transportation, the owner of the barges, were working to remove the seven barges that came to rest against Emsworth Locks and Dam, which lies on the Ohio River directly downstream from Pittsburgh, Vernon said. Those operations are expected to continue through noon or slightly past, she said.

Removal of the other barges, including one that ended up at Dashields Locks and Dam, which lies just upriver near Pittsburgh, is now in the hands of Campbell Transportation, she said.

"The barges came to rest on our facility," Vernon said. "Our main priority is to work with our partners to ensure that this process moves safely and that our personnel involved remain safe."

Campbell Transportation said in a media update on Monday the one barge declared missing earlier was "presumed to be found" using sonar technology in Dashields Pool, which lies next to the dam, after the USACE said they believed it had sunk.

Of the vessels that broke loose, 23 were loaded open hopper barges and three were empty, according to the company. As of noon on Monday, 17 of the barges were secured.

A spokesperson for Campbell Transportation did not respond to multiple requests for comment.

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Are the bridges open?

The McKees Rocks, Neville Island and Sewickley bridges were initially closed "out of caution" while the barges drifted loose, Alexis Campbell, a spokesperson for the Pennsylvania Department of Transportation, told USA TODAY in an email. All three were reopened within hours the next morning.

The navigation channel remains closed, Vernon said.

The Captain of the Port of Pittsburgh put a safety zone into place between the two dams on Monday, barring navigation and transport in the stretch of river, the USACE said in a Facebook post. The channel can't reopen until the safety zone is lifted, according to Vernon.

With 184 million tons of cargo transported on the Ohio River each year, according to the Ohio River Valley Water Sanitation Commission, the closure of the channel could impact the flow of Pittsburgh's maritime activity.

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Have other barges broken loose on the Ohio River?

The 26 barges are far from the first the first to break loose on the Ohio River. Early last month, the USACE in Louisville reported that two barges loaded with coal were pinned against the McAlpine Locks and Dam, around 4 miles down the river from downtown Louisville, causing it to shut down, while eight other barges were pinned against the Louisville and Indiana Railroad bridge pier.

The incident came almost exactly one year after another 10 barges broke free from their tugboats on the same stretch of river near Louisville, the Louisville Courier Journal, part of the USA TODAY Network, reported.

One of the barges, which carried 1,400 tons of methanol, was submerged at the McAlpine Dam, according to the Kentucky Energy and Environment Cabinet. The barges also carried soy, corn, and other products.

Officials took more than a week to fully unload the methanol before the barge was pulled off the structure. The incident stirred concerns that methanol could have escaped into the river and entered local water sources, but authorities said tests of the water did not detect the chemical.

Cybele Mayes-Osterman is a breaking news reporter for USA Today. Reach her on email at [email protected]. Follow her on X @CybeleMO.

Contributing: Eduardo Cuevas