A construction worker on the Highway 520 Montlake Project was killed Wednesday when he was pinned between an 11,000-pound steel beam and the flatbed trailer of a semi, according to the Seattle Fire Department.

On-site workers were able to free the man before medics arrived, but the 45-year-old victim, whose identity was not immediately made public, died en route to a hospital, said Fire Department spokesperson Kristin Tinsley.

The worker was part of the crew assigned to the $455 million project to reconstruct Highway 520.

Washington State Department of Labor & Industries spokesperson Dina Lorraine said the state has opened an investigation into the incident and the two contractors on the project, Graham Construction and American Bridge.

She said the agency does not yet know the name of the worker.

The investigation will look into what caused the incident and whether there were any workplace safety and health violations that contributed to it, Lorraine said. If safety violations are found, citations and fines can be issued.

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The department has up to six months to complete the investigation and report its findings.

Graham Construction and American Bridge Company deferred questions to the Washington State Department of Transportation.

“Our thoughts are with this worker’s family and friends as well as all the workers on the job. In the wake of on the job incidents like this, it is customary for L&I to conduct an investigation,” Hannah Britt, WSDOT assistant construction communications manager for the project, said in an email.

This is the second time someone has died while working on the Highway 520 megaproject, which is expected to be completed in 2023 to 2024. Joe Arrants, a carpenter from Burien, fell while building an elevated roadway near the east shore of Lake Washington in 2015, after his safety-harness cable broke. That segment of the bridge was built by a different contractor, Kiewit-General-Manson.

Seattle Times transportation reporter Mike Lindblom and The Associated Press contributed to this article.