Wednesday’s blast at a BP refinery that killed 15 people and injured at least 100 is called the worst gas-and-chemical industry accident in 15 years.

The refining industry is the main priority for Texas City. The facility of London-based BP employs 1,800 people and sprawls across 1,200 acres. It processes 433,000 barrels of crude oil a day, producing 3% of the U.S. supply of gasoline.

After Wednesday’s explosion BP also seems to be the main killer.

Allan Jamail, an official with Pipefitters Union Local 211 in Houston, accused the company of engaging nonunion contractors who "aren’t as well-trained".

BP officials denied that using contractors might be violation of security confirming that the safety record "has improved enormously."

At a news conference at the Texas City Hall Lord Browne, chief executive officer of London-based BP, called the incident "the worst tragedy that I’ve known" in 38 years with the company.

He said that company officials were not sure about the causes, so the company is said to involve experts from other refineries to define what happened.

Three separate investigations of government and independent agencies are conducted in order to clear up the incident.