Two men who fell seriously ill following a clinical drugs trial remain in a critical condition but four others are showing signs of improvement.
The six are being treated at Northwick Park hospital
All six are still in intensive care in Northwick Park Hospital, north-west London, after falling ill on Monday.
TeGenero, which manufactures the anti-inflammatory drug, says it has apologised to the men's families.
Scotland Yard said officers are talking to the Medicines and Healthcare products Regulatory Agency and doctors.
TeGenero described the reactions as "shocking developments" and said the new medicine had showed no signs of problems in earlier tests.
Clinical Director of Intensive care Ganesh Suntharalingham at Northwick Park Hospital said: "Of the six patients admitted to critical care, the four who are seriously unwell are continuing to show signs of improvement but it is still early days.
"The other two men remain critical and it could be a while until they show significant change."
One of the critically ill men has been named as student Ryan Wilson, 21, of Highbury, north London.
Another who was taken seriously ill has been confirmed as a New Zealander. The New Zealand High Commission said he was "conscious and has spoken to hospital staff".
Went down like dominoes
Myfanwy Marshall, 35, whose boyfriend is critically ill, said the normally healthy 28-year-old's face was so puffed, he "looks like the Elephant Man".
It was the first time the drug TGN1412, designed to treat conditions such as rheumatoid arthritis and leukaemia, had been tested on humans.
Within hours of taking it on Monday, the six young volunteers had to be admitted to intensive care.
They "went down like dominoes", according to Raste Khan, 23, one of two trial volunteers who escaped unscathed after being given a placebo.
Myfanwy Marshall: Worried about her boyfriend
He told the Sun newspaper: "They began tearing their shirts off complaining of fever, then some screamed out that their heads felt like they were going to explode.
"It was terrifying because I kept expecting it to happen to me at any moment. But I felt fine and I didn't know why."
The Medicines and Healthcare products Regulatory Agency is looking at whether the reaction was caused by a manufacturing problem, contamination, a dosing error or whether it was some "completely unanticipated side-effect of the drug in humans".
Health minister Jane Kennedy described the events as "wholly unexpected", saying the top priority was to protect patients and then establish the causes.
'Shocked and devastated'
American company Parexel, which ran the trial, said it had followed recommended guidelines.
Chief scientific officer Thomas Hanke added the company's first concern now was making sure the patients got the best treatment possible and to support the families.
He said: "They were shocked, devastated. We deeply understand that they are. We are devastated at these shocking developments which we were not anticipating."
But the families said they had been given "mixed messages" during two meetings with the companies.
A Metropolitan Police spokeswoman said it was "more than likely" officers had visited the hospital, adding that police were keeping a close eye on the case.