Tests to find out how an Angolan man died while being deported from the UK have proved inconclusive, police said.
Jimy Mubenga, 46, collapsed on a plane at Heathrow airport as he waited to be taken back to his homeland earlier this week.
Eyewitnesses told the Guardian newspaper that he was held down by security guards on the aircraft and had complained of breathing difficulties.
The Met police today confirmed their homicide unit had taken command of the investigation after it was initially dealt with by officers from Heathrow CID.
The Home Office said they couldn't comment on claims that Mr Mubenga had been restrained before his death.
Chairman of the Home Affairs Select Committee Labour MP Keith Vaz has said he will write to Home Secretary Theresa May about the incident.
He told the Guardian: "The use of excessive force in deportations is dangerous and unacceptable.
"If as eyewitness reports suggest, Mr Mubenga was complaining of breathing difficulties, questions must be asked as to why help was not called for sooner."
Mr Mubenga became unwell while under the escort of three civilian security guards on a British Airways flight that was preparing to leave Heathrow for Angola shortly before 8.30pm on Tuesday.
He was taken to Hillingdon Hospital, in Uxbridge, but was pronounced dead shortly after arrival.