HOUSTON -- An industrial crane collapsed and killed four people at an east Houston refinery Friday afternoon, KPRC Local 2 reported.

Officials said the large crane fell over at the LyondellBasell refinery on Lawndale Street at Highway 225 shortly after 1:30 p.m., knocking over two other cranes as it came down.

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A LyondellBasell spokesman said four died and seven others were injured.

Two of the injured were flown by Life Flight helicopter to Memorial Hermann Hospital. They remained hospitalized Friday night with non-life-threatening injuries.

Witnesses said an alarm sounded, and employees ran to a lunch tent that was designated as an evacuation area. However, the crane toppled on top of the tent, killing some inside.

"It sounded like a building -- sounded like a building fell. I looked back -- I was on my way to evacuate the plant and I looked back and I seen the arms coming down. After that, I just seen a lot of black smoke," worker Stacy Davis said. "I stopped in my tracks. We was in shock. I was in the middle of the street. I didn't know what to do. I wanted to cry. I believe some people got killed. It was so big and so loud and I seen it come down."

A LyondellBasell official said all of the victims, including the fatalities, were contract employees. Their names have not yet been released.

"We were in the process of extensive turnaround activity, so we had more than our normal number of contract employees on site," said Jim Roecker, LyondellBasell's vice president of operations. "It's a very sad day for us here at the refinery."

Company officials said the crane was one of the world's largest and was being leased for maintenance work when it collapsed. Deep South Crane and Rigging was contracted to work with the crane during the turnaround.

A test was performed on the crane a couple of days ago in which it safely picked up 800,000 pounds. The equipment can pick up 1 million pounds, according to authorities.

Officials said they do not yet know what caused the crane to collapse or what the workers were doing at the time of the accident.

"As far as we're concerned, there were no planned lifts with that crane for today. We're going to be working with the contract company that operates that crane to determine if they were involved in any type of activity at the time of the incident. We certainly did not know anything that was planned," Roecker said.

Everyone has been accounted for at the plant, according to company officials.

About 70 Houston firefighters responded to the accident. Some people were rescued from underneath the wreckage.

As many as 4,500 people, including thousands of contract workers, were at the plant at the time of the accident.

"Periodically, we take our operating units down to perform maintenance activities. In this case, it's about once every five to six years to do liability improvements and upgrades. And that takes anywhere normally from 45 to 60 days in the case of these particular units that we were about to turn around," Roecker said.

The crane was delivered in pieces and assembled on site about a month ago. It was brought in to remove large drums from inside a coking unit whose roof had been cut off to allow the crane access, Roecker said.

The Houston refinery is one of the world's largest for processing high-sulfur crude oil. The facility itself covers about 700 acres along the Houston Ship Channel at the city limits of Houston and Pasadena.

The Occupational Safety and Health Administration was called to the scene to investigate.

Lyondell Chemical, a U.S. company, and the Dutch firm Basell were rivals until they announced a $12.1 billion deal last July to create one of the world's largest chemical companies.

On the chemical side, Lyondell produces ethylene, a crucial precursor to a range of other chemicals, as well as propylene oxide, which is also used in producing a variety of chemical products. Basell focuses on polyolefins, common types of plastic.

Deep South Crane and Rigging's Statement

"We have experienced a significant accident involving Deep South Crane employees and equipment at the Lyondell-Citgo facility in Pasadena, Texas. We have few details at this time.

"Our primary concern now is the identification of injured employees and the notification of families. We are taking every measure to ensure that the injured employees receive the best possible medical attention. Our thoughts and prayers are with our employees and their loved ones.

"At this point, we have few details on what actually happened and we are trying to gather information. We will use this information to conduct an investigation to determine the root cause, correct it and ensure that this type of tragedy does not occur again. We will cooperate fully with all investigations that may arise from this tragic incident.

"We will provide information as we gather and verify it. In the meantime, we ask for your prayers and patience in this difficult time."