The British government on Friday defended the country's security services against criticism that they missed signs which might have helped prevent the brutal murder of a soldier on a street of south-east London.
MI5 now faces a House of Commons inquiry after it was confirmed the two terror suspects arrested over his murder were known to the national security agency.
But communities secretary Eric Pickles said it was impossible to control everyone all the time.
"Peers and MPs will do a thorough investigation in terms of what the security forces knew but I've seen experts on security explaining how difficult it is in a free society to be able to control everyone," Pickles was quoted as saying by the BBC.
Drummer Lee Rigby, a 25-year-old soldier from the 2nd Battalion Royal Regiment of Fusiliers, was hacked to death by the two Islamist extremists near Woolwich barracks on Wednesday.
Both Londoners of Nigerian descent were caught on camera-phones and have been identified as 28-year-old Michael Adebolajo and 22-year-old Michael Adebowale.
Adebolajo, from a Nigerian churchgoing family who converted to Islam after college, had complained of harassment by MI5 in the last three years after he came to the intelligence agency's attention.
He assumed a Muslim name of Mujahid, which means one who engages in jihad, and began attending demonstrations of the now-banned Islamist group al Muhajiroun in the UK.
Shortly after the attack, Adebolajo was filmed by a passer-by, saying he had carried out the attack because British soldiers killed Muslims every day.
Holding a knife and machete in his bloodstained hands, he said, "We swear by almighty Allah we will never stop fighting you. We must fight them as they fight us. An eye for an eye, a tooth for a tooth."
Whitehall sources did not deny reports that one of the suspects was stopped while trying to travel to the war-torn east African country of Somalia, feared by counter-terrorism officials to be a training ground for violent jihadists.
Both suspects, shot by police shortly after the incident, remain in separate hospitals and are too badly injured to be questioned at present.
Detectives investigating Rigby's death also arrested a 29-year-old man and woman on suspicion of conspiracy to murder the soldier, suggesting there may have been a wider plot to carry out the attack.
It is understood that a flat in Greenwich raided by police was occupied by the girlfriend of Michael Adebowale.
British Prime Minister David Cameron had said on Thursday that Parliament's intelligence and security committee would examine the wider role of the police and MI5, an inquiry that is expected to address any lessons that may need to be learned after counter-terrorism officials decided not to monitor the suspects.
Police have deployed 1,200 extra officers amid fears of a backlash in London and dozens of Islamophobic incidents were reported in the wake of the murder, including attacks on four mosques.