STOW, MASS. (WHDH) - Several members of the Massachusetts State Police were injured in an explosion during a June safety course at the Department of Fire Services in Stow, officials said.

The incident took place on June 20 during a lesson carried out by a private company showing first responders the dangers of illegal drug labs. Members of the State Police and state hazmat personnel were also involved.

A newly released video shows two workers mixing chemicals while wearing heavy protective gear. The chemicals suddenly exploded during the video.

A trainer without protective gear was standing just feet away and others were watching out of view of the camera.

In a letter to the training company, the Department of Fire Services wrote, “… this incident has led to five individuals being evaluated at a local area hospital and a total of twelve individuals complaining of various injuries.”

Fire Services also said the accident damaged thousands of dollars of equipment.

News of the incident surfaced within weeks of a State Police trainee dying after suffering serious injuries during a training exercise.

“There must be something that went wrong. When we have an incident that transpires in training, and it’s not a scheduled incident, then that is something that has gone awry,” said 7News Law Enforcement Expert Tom McGhee.

An independent investigator has been appointed to review the trooper trainee’s death.

McGhee said the June 20 incident should be investigated, as well, even though the consequences were much less serious.

“We need to investigate. We need to recognize what’s going on. All training is going to be scrutinized, I would imagine, and this is just another incident — unfortunate timing — but just another incident that will be under review,” he said.

The company that conducted the chemical exercise replaced the thousands of dollars’ worth of equipment damaged without admitting blame for doing anything wrong. The company has not responded to a request for comment.

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