A solicitor representing one of the men said it was not clear if successful animal tests had been held before the tests were conducted.
Anne Alexander, whose 29-year-old client is on a life support machine, claimed: "There is confusion about whether the drug had actually been tested successfully and safely on animals before the tests on these volunteers."
Rhaste Khan from Barry, near Cardiff, who was lucky enough to have been given a placebo said he felt OK after being given his injection but that the others on the trial began falling "like dominos".
And Mr Khan had told the Sun newspaper: "Some screamed out that their heads felt like they were going to explode."
It was the first time that the anti-inflammatory drug TGN1412, designed for the treatment of conditions such as rheumatoid arthritis and leukaemia, had ever been tested on humans but American company Parexel, which ran the trial, said it had followed the recommended guidelines.
TeGenero, the company, which makes the drug, said it apologised to the sick men's families and said the medicine had showed no signs of problems in earlier tests.
Scotland Yard said police officers are talking to the Medicines and Healthcare products Regulatory Agency and doctors as well.
