One of the men accused of killing a British soldier on the streets of London had appeared in the dock in Kenya on terror charges three years ago.
Two UK newspapers have reported that Michael Adebolajo, who was filmed with blood on his hands after the attack in London, was held by Kenyan police along with a five other "radicalised" Muslim youths.
The papers claim that the youths had been trying to join the terror group al-Shabaab in neighbouring Somalia.
They appeared in court on suspicion of planning to fight for terrorists.
Adebolajo was deported to Britain, after which he allegedly came to the attention of MI5.
The Mail on Sunday claims that British intelligence officers pegged him as a potential informant, offering him money and a mobile phone.
A close friend of Adebolajo told the paper that officers asked him to spy on a group of Muslims but he rebuffed their overtures.
The Sunday Telegraph reported that two months before Adebolajo was detained in Kenya, the head of MI5 gave warning that Britons were training with terorists in Somalia.
The paper said that Michael Adebowale, the other suspect in Drummer Lee Rigby's murder, had been held by police in London two months ago.
The news came as British police arrested three more suspects in connection with Rigby's murder.
Scotland Yard said counter-terrorism officers arrested two men, aged 24 and 28, at a residential address in southeast London. A third man, 21, was arrested separately on a London street at the same time.
Police said they used a stun gun on two of the suspects. All three were detained on suspicion of conspiracy to commit murder.
Officers have already detained several others in connection with the murder of Rigby, who was hit with a vehicle then repeatedly stabbed with knives while walking outside the Royal Artillery Barracks in the Woolwich, south London, on Wednesday afternoon.
The horrific scenes were recorded on witnesses' phones, and a video has emerged in which one of the two suspects made political statements and warned of further violence as the dead soldier lay on the ground behind him.
Adebowale and Adebolajo were shot by police who arrived at the scene minutes later. They are under guard in two separate hospitals.
Three other people were arrested Thursday in connection with the probe. Two women were released without charge, and a 29-year-old man has been bailed pending further questioning.
Another man was arrested on suspicion of unspecified terrorism offenses late Friday immediately after he gave a BBC interview detailing the background of one of the main suspects. The man, identified by the BBC as Abu Nusaybah, was arrested on BBC premises and remains in custody.