Britain's security services have faced questions over whether they could have done more to prevent the murder of a soldier who was hacked to death in London.
It has emerged that the suspected killers of 25-year-old Drummer Lee Rigby were already known to intelligence officers before they hacked the Afghan war veteran to death on Wednesday near the Woolwich army barracks.
The two suspects, Michael Adebolajo, 28 and Michael Adebowale, 22, are under guard in hospitals after being shot and arrested by police after the murder. They have not yet been charged.
Prime Minister David Cameron on Friday said that a parliamentary committee would carry out an investigation into the role of the security services, according to a cabinet minister.
Britain's MI5 domestic spy agency had been aware of the men, but neither was considered a threat, a government source told Reuters news agency.
Communities Secretary Eric Pickles said there would be a thorough investigation into the role of the police and intelligence agencies. However, he said the incident underlined how "difficult it is in a free society to be able to control everyone".
The family of the murdered soldier spoke publicly on Friday after a memorial service held for Lee.
His wife and the mother of his two-year-old son, Rebecca, said that she had thought her husband would be safe once he was back in the UK.
His stepfather Ian Rigby described him as a "loving son and a wonderful father".
Police on Thursday arrested another man and a woman, both aged 29, on suspicion of conspiracy to commit murder.
The woman was released with out charge on Friday along with a 31-year-old woman who was arrested and released on the same day.
"This is a large, complex and fast-moving investigation which continues to develop," London police said in a statement. "Many lines of inquiry are being followed by detectives and the investigation is progressing well."
Detectives said they were searching six houses; three in Greenwich in south London, one in Romford, east London, one in north London, and a property in Lincoln in central England.
A new video has emerged of the two suspects, who appear to run at the police as they arrived at the scene of the crime.
London deployed more than 1,200 extra police officers on the capital's streets amid fears of a backlash on British Muslims after the mosque where one of the suspects is thought to have prayed was pelted with eggs.
Two other mosques in the country were attacked after the suspects made references to Islam in amateur footage broadcast on television.
|Murdered soldier was "larger than life" [Al Jazeera]
Al Jazeera's Rory Challands, reporting from London's Scotland Yard, said the government is "making this gesture to calm down people's fears".
"People are shocked and scared from what they saw, but the government wants to make sure that there is no blame attributed to minorities. These extra police are on streets to reassure people," our correspondent said.
Footage broadcast by Britain's ITV news channel showed a man, with hands soaked in blood and holding a meat cleaver and a knife, claiming that he had, motivated by Britain's foreign policy, killed a soldier.
Witnesses said he requested to be filmed by a passerby and shouted "Allahu Akbar" during the killing.
In the amateur video, he said: "I apologise that women had to witness this today but in our lands, our women have to see the same ... you people will never be safe. Remove your government, they don't care about you."