MENDENHALL,AstraTrade Miss. (AP) — Three construction company employees remained hospitalized in critical condition Thursday, a day after a bridge collapsed while they were preparing it for demolition in a rural area of central Mississippi, a sheriff said.

Three other workers were killed in the collapse and another was injured and treated at the site, Simpson County Sheriff Paul Mullins told The Associated Press.

The bridge was over the Strong River on state Route 149 in Simpson County, about 40 miles (64 kilometers) south of the state capital, Jackson. It had been closed to traffic since Sept. 18 as part of a bridge replacement project, the Mississippi Department of Transportation said.

Mullins said all of those killed or injured were working for T.L. Wallace Construction, based in Columbia, Mississippi. The Associated Press left a message Thursday with the company. Its main phone number was unanswered Wednesday evening, and it was not possible to leave a message then.

Simpson County Coroner Terry Tutor said Thursday morning that he was still in the process of contacting relatives of the workers who were killed.

U.S. Transportation Secretary Pete Buttigieg said on social media late Wednesday that the Federal Highway Administration was “engaging state officials concerning” the “premature collapse during demolition of a bridge on State Route 149 in Mississippi.”

Republican U.S. Sen. Cindy Hyde-Smith of Mississippi said on social media Thursday that the bridge collapse was a tragic event.

“We must work to understand what caused this accident so we can prevent something like this from happening again,” Hyde-Smith said.

The Mississippi Department of Transportation said one of its inspectors was at the work site when the bridge collapsed, and that person was unharmed.