A collapsed crane is visible Friday at the LyondellBasell Houston Refinery. One of the nation's largest mobile cranes collapsed at a Houston oil refinery Friday, killing four workers and injuring seven others in the latest of several fatal accidents that have raised concerns about the safety of construction cranes.

Houston ? Federal investigators on Saturday began trying to figure out why one of the world’s largest mobile cranes toppled over, killing four contract workers and injuring seven others.

Officials said it could take time before knowing what caused the 30-story-tall crane to collapse Friday at a LyondellBasell refinery in Houston, the latest of several deadly crane accidents around the country.

The massive crane, capable of lifting 1 million pounds, was owned by Deep South Crane & Rigging, which Saturday released the names of its four workers killed in the accident.

“We wish we had all of the answers on what happened and why – but we do not, and speculating on cause would not resolve anything,” the company said in a statement. “But we are actively working to find those answers.”

The four men killed were: Marion “Scooter” Hubert Odom III, 41, of Highlands; John D. Henry, 33, of Dayton; Daniel “DJ” Lee Johnson, 30, of Dayton; and Rocky Dale Strength, 30, of Santa Fe, Texas.

At the LyondellBasell refinery, company officials said they were trying to restore normalcy. The refinery brought in grief counselors and will hold a series of safety meetings to address concerns about the accident starting Monday, said David Roznowski, a company spokesman.

“This is a real blow to our refinery team, and it will take some time to recover from this,” said Roznowski.

Investigators with the Occupational Safety and Health Administration began their formal accident investigation early Saturday, Roznowski said.

Cameras are mounted around the plant, and refinery officials said the company hopes the video will help determine what happened.

Two of the injured workers remained in Houston hospitals Saturday. Their injuries were not life-threatening, Roznowski said.

Two other injured workers were taken to a hospital and have since been released. Three others were treated and released at the scene, fire officials said.

The massive crane fell Friday afternoon with enough force to lift workers off the ground, and toppled across another smaller crane and a tent where workers were eating lunch.

Crane safety has been getting extra scrutiny in recent months because of an alarming number of crane-related deaths in places such as New York, Miami and Las Vegas.

The crane failed and collapsed during maintenance, LyondellBasell officials said.