Three men who were arrested on suspicion of conspiracy to commit murder in connection with the killing of Drummer Lee Rigby have been released on bail, the Metropolitan Police said today.

The trio, aged 21, 24 and 28, were arrested in south-east London on Saturday, three days after the 25-year old soldier was hacked to death in Woolwich. They have been bailed to return to a south London police station.

Meanwhile, two people have been arrested on suspicion of arson after a fire was discovered shortly before 10pm yesterday at the Grimsby Islamic Cultural Centre, in Weelsby Road.

The fire was extinguished and no one was injured, Humberside Police said.

The two murder suspects, Michael Adebolajo, 28, and Michael Adebowale, 22 - both Britons of Nigerian descent - remain in custody in hospital in a stable condition after being shot and wounded by police at the scene after the killing.

It emerged yesterday that Mr Adebolajo was detained by Kenyans for suspected extremist activity in 2010 and later deported back to Britain.

Kenyan counterterrorism police arrested Mr Adebolajo after he allegedly attempted to join up with al-Shabaab Islamist militants in neighbouring Somalia. He appeared in court under a different name, Michael Olemendis Ndemolajo. The foreign office said it had provided him with “consular assistance” after he was held, “as is normal for British nationals detained”.

The arrest raises questions as to why the security services did not monitor him more closely on his return to the UK, given his apparent unsuccessful attempt to fight alongside jihadist extremists. Kenyan officials indicated yesterday they had briefed their British intelligence counterparts about his radical activities.

“Kenya’s government arrested Michael Olemendis Ndemolajo. We handed him to British security agents in Kenya and he seems to have found his way to London and mutated to Michael Adebolajo,” a Kenyan counterterrorism spokesman, Muthui Kariuki, told the Associated Press. He added: “The Kenyan government cannot be held responsible for what happened to him after we handed him to the British authorities.”

Mr Adebolajo was arrested in November 2010 together with seven other young Islamists. Kenyan police captured the group after they travelled by speedboat from the Indian Ocean island of Lamu to Kizingitini, on Pate Island, 40 miles from the Somali border. The police acted following a tip-off.

Two of those apprehended were teenage schoolboys who had apparently been radicalised during regular visits to a mosque in Mombasa.

The Kenyan authorities later released the group without charge. Mr Adebolajo told local media the police had severely mistreated him. According to family members he was beaten, and threatened with rape and death. There is evidence he subsequently lodged a complaint. Kenyan officials deny wrongdoing.

His family say he returned to the UK a changed and embittered man. His brother-in-law, who did not wish to be identified, said: “He was very withdrawn. And he saw everyone in authority as being his torturer.”

At this point, 2½ years ago, according to relatives, MI5 began to take an increased interest in Mr Adebolajo. “They had been interested in him for years,” the brother-in-law said.

“MI5 asked me about him in 2009. But they began pestering him after he came back from Kenya. They would be banging on his door or approaching him in the street. It was happening constantly; it was every couple of days. At first they wanted information, then they wanted him to spy for them. I assume they wanted him to spy on Muslims.”

It is unclear why Mr Adebolajo did not face prosecution on his return to the UK, if there was evidence to support allegations that he went to Somalia to seek training with al-Shabaab.

MI5 will be questioned this week about its alleged connections with Mr Adebolajo and Michael Adebowale, the second suspect in the murder of Lee Rigby. Members of the Intelligence and Security Committee, the panel of MPs and peers that provides oversight of the UK’s intelligence agencies, will ask what exactly the agency knew about their links to extremism.

Agencies