In a statement to ABC News, BP called the chemical "one of the most well-studied dispersants" and said it chose Corexit in part because it could "get a sufficient supply to meet our needs on short notice."
For weeks, the company assured that using the dispersant was safe, with officials describing them like soap suds. But today, others painted a far more sinister picture.
"Any living organism that contacts this stuff, particularly the mixture of dispersant and oil, is at significant risk of acute mortality," said marine biologist Rick Steiner.
In fact, EPA testing released today indicates that where the dispersant had been used, 25 percent of all organisms living at 500 feet below the surface died.
Markey called BP's use of the untested dispersant a "science experiment."
"It is very important that we not allow BP to conduct a science experiment in the oceans of the United States without the closest supervision in terms of the amount of toxic material that is put into the ocean as a way of controlling this spill," he said.
Taken together, the company's resistance to transparently supply footage of the leak and use of a toxic dispersant indicated to him that BP could not be trusted.
"I think now we're beginning to understand that we cannot trust BP. People do not trust the experts any longer," said Markey. "BP has lost all credibility. Now the decisions will have to be made by others, because it's clear that BP has been hiding the actual consequences of this spill."
ABC's Sunlen Miller contributed to this report.