Investigators will return to the site of a massive explosion Friday.

Hazardous materials crews on Friday will use a robot to detonate some of the dangerous chemicals that injured four workers. 

The four people suffered critical injuries following a chemical explosion at the DOW Chemical Company in North Andover.

The incident happened at around 2:30 p.m. Thursday at the plant on Willow Street. The explosion was so powerful, neighbors said they were able to feel it.

"I heard a loud boom, the house shook a little bit," said Kaela Hennessey, who lives nearby. "My dog went a little crazy and I looked out my back window to see if there was an accident on Route 125."

Four employees were injured and rushed to the hospital, where they were decontaminated in the parking lot before they were brought inside. Within an hour, three of the patients were airlifted to Boston for more intense trauma care. 

On Thursday afternoon, Lawrence General Hospital released a statement about the explosion patients: 

Due to a chemical explosion, we received four critical patients with shrapnel injuries and burns. The patients were decontaminated outside the hospital, per our policy. Three of the patients where (sic) stabilized and transferred to Boston tertiary medical centers (sic). The fourth patient is staying here at Lawrence General.

The hospital also said, "All four patients were assessed in critical condition, which, according to hospital policy, means ‘Vital signs are unstable and not within normal limits. Patient may not be conscious; indicators are unfavorable or questionable.’"

DOW develops materials that are used in LED light manufacturing. According to a source there was an explosion in a laboratory involving chemicals. Doctors said they were told the victims came in contact with the chemical trimethal aluminum. 

"It’s a chemical that’s an irritant to the skin and anything it came in contact with," said Dr. George Kondylis of Lawrence General Hospital.

Employees at the site who were not injured were evacuated and boarded school buses to leave the area. Firefighters said people who live nearby are safe and should not be worried about chemicals in the air.

In October of 20134, a DOW Chemical employee was killed in an explosion in a lab. The fire marshal said different chemicals were involved in both explosions and do not appear to be connected.

OSHA was on the scene Thursday to determine the stability of the building.

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