The Foreign Office is investigating reports that two UK terror suspects died in a US drone strike in Pakistan.
Ibrahim Adam, 24, and Mohammed Azmir, 37, both from east London, are said to have been killed in Waziristan on the Afghanistan border.
Mr Adam's father confirmed his son had been killed by a US unmanned aircraft.
A Foreign Office spokesman could not confirm the deaths but said: "We are aware of reports and are looking into them further."
Mr Adam, from Barkingside, had been on the run from the UK authorities since absconding from a control order in May 2007.
His brother, Anthony Garcia, was jailed for life in April 2007 for his part in a fertiliser bomb plot in the UK.
Mr Adam was made subject to a control order after being stopped on the way to Syria.
British authorities said he was planning to travel to Iraq or Afghanistan for jihadist training or to fight Western forces.
Mr Azmir, a father of three, who was born in Sheffield and lived in Ilford, was made subject to a Treasury order freezing his assets in February 2010.
He is not currently on the Treasury's list of people and organisations subject to financial sanctions.
It is believed that Mr Azmir's brother, Abdul Jabbar, 32, was killed in an earlier American drone attack in Pakistan in September last year.
An Azmir family friend, who did not want to be named, said: "They have taken it very badly. This is the second son who has been killed in a drone strike."
The Home Office declined to comment.