Investigations into how a drugs trial left six men seriously ill - two of them critical - are continuing.
The six are being treated at Northwick Park hospital
Police and regulators are probing whether problems were caused by errors or were an unpredictable side-effect.
Staff from Parexel, the firm that carried out the trial at Northwick Park Hospital in north-west London, insist the right procedures were followed.
Four men left seriously ill by Monday's trial have regained consciousness. The other two remain critically ill.
But medics say they are beginning to respond to treatment.
Four of the men suffered multiple organ failure within hours of taking the drug TGN1412 on Monday.
The Medicines and Healthcare products Regulatory Agency is investigating whether the reaction suffered by the men was caused by a manufacturing problem, contamination or a dosing error.
They have seized documents and sealed off offices at the hospital research unit.
Scotland Yard said officers were talking to the MHRA and doctors.
Herman Scholtz, head of Parexel, praised staff's swift response when volunteers suffered a reaction during tests of an anti-inflammatory drug.
In a statement posted on the website of US-based Parexel Dr Scholtz said: "An initial review at the site has shown that best practices were followed.
"We remain in close and constant contact with the Northwick Park Hospital concerning the medical status of the six volunteers."
On Friday, local Labour MP Barry Gardiner met the North West London Hospital Trust chief executive Mary Wells, and said there was no better place for the patients to be treated.
Mr Gardiner said: "MHRA has sealed the offices of Parexel, bagged and taken away all the relevant information, is conducting interviews with Parexel and with their staff."
He added it was unclear how long the inquiry by the UK medicines watchdog would take.
TeGenero, which manufactures the drug, apologised to the sick men's families and said the medicine had shown no signs of problems in earlier tests.
The company's chief scientific officer, Thomas Hanke, said he and his colleagues were "devastated" by what had happened.
He said TGN1412 had been tested extensively in laboratories and on rabbits and monkeys with no adverse effects and no drug-related deaths.
It was the first time the drug, designed to treat conditions such as rheumatoid arthritis, leukaemia and multiple sclerosis, had been tested on humans.
Dr Ganesh Suntharalingam, head of intensive care at Northwick Park, said the two men worst affected by the trial were showing signs of recovery but they remained under sedation.
He added: "Some of them have made noticeable progress in response to our treatment and we have been able to reduce the amount of organ support that is required.
"However, it is early days and they will clearly still continue to need specialist observation for some considerable time.
"There are also some very early signs of response to treatment in the most critically ill patients but I must stress that their condition remains very serious and complex and it wouldn't be sensible to comment on prognosis."