LONDON — British Prime Minister Theresa May on Tuesday ordered a public inquiry into how contaminated blood was used to treat thousands of people in the 1970s and ’80s, killing at least 2,400, according to officials.

Thousands of British public health patients — many of them hemophiliacs — were infected with the HIV virus or hepatitis C through tainted blood. Authorities looked into the infections, but campaigners and lawmakers say they did not go far enough to find out what happened.

Downing Street said Tuesday the new probe would aim to “establish the cause of this appalling injustice.”

“The victims and their families who have suffered so much pain and hardship deserve answers as to how this could possibly have happened,” May said.

The announcement came after leaders of six political parties signed a letter calling for a fresh investigation to encompass allegations of a cover-up.

Labour lawmaker Diana Johnson called it “the worst treatment disaster in the history of the National Health Service” and said officials had failed to consider evidence of possible criminal activity.

Minister of State for Health Philip Dunne said the new inquiry would have the ability to bring criminal charges.

The contaminated blood was linked to supplies of a clotting agent called Factor VIII, which British health services imported from the United States. Some of the products turned out to be infected. Some of the plasma used to make the blood products were traced to high-risk donors including U.S. prison inmates who were paid to give blood samples.

Most of those affected were infected with hepatitis C. Of the 1,200 people infected with the HIV virus, fewer than 250 still are alive, according to the Hemophilia Society.

In April, former heath secretary Andy Burnham told Parliament the scandal “amounts to a criminal cover-up on an industrial scale.” He cited altered medical records and gave examples of tests being conducted on patients without their knowledge or consent.

Results from those tests were withheld for years, even decades, he said.

Former Prime Minister David Cameron apologized to the victims’ families in 2015 after a report on the infected blood was issued. Victims and families of those who died complained that the report was a “whitewash.”