LONDON -- Both the suspects accused of butchering a British soldier during broad daylight on a London street had long been on the radar of Britain's domestic spy agency, though investigators say it would have been nearly impossible to predict that the men were on the verge of a brutal killing.
Still, counterterrorism officials said they are reviewing what -- if any -- lessons can be gleaned from the information they had leading up to the slaying Wednesday.
Authorities in the U.S. have similarly pledged to review their procedures in the wake of the Boston Marathon bombing, with the Boston police commissioner saying cities should consider deploying more undercover officers and installing more surveillance cameras.
The British review comes amid an outpouring of grief over Wednesday's slaughter of 25-year-old Lee Rigby of the 2nd Battalion the Royal Regiment of Fusiliers. Rigby, who had a two-year-old son, had served in Afghanistan. Detectives say they do not believe the attackers knew him or that he was specifically targeted, but they are still investigating.
"We are looking at decisions that were made and reviewing whether anything different could have been done," said a British counterterrorism official who spoke on condition of anonymity because he was not authorized to speak about the investigation. "But you can't put everyone under surveillance who comes on to the radar."
Prime Minister David Cameron said Thursday the intelligence and security committee would review the work of agencies such as Britain's domestic spy agency, MI5, in the wake of the attack "as is the normal practice in these sorts of cases."
In Britain, security officials operate under the "principle of proportionality," which means there needs to a compelling reason before any type of surveillance is undertaken.
Surveillance can range from watching a person's movements to intercepting phone calls and electronic communication. The greater the level of intrusion into a person's privacy, the higher the level of government approval needed.
Although British police have not named either suspect -- both are recovering from their injuries after being shot by police after the killing -- they had been known to law enforcement officers for as long as six years, the counterterrorism official said.
One of the men -- seen wielding a bloody butcher knife in video footage after the attack -- was identified as Michael Adebolajo by two Muslim hardliners, Anjem Choudary and Omar Bakri Muhammad, who said they knew him as a Muslim convert who took part in demonstrations by a banned radical group.
A man who described himself as a friend of Adebolajo said the suspect was being "basically harassed" by security services.
Abu Nusaybah told the BBC on Friday that Adebolajo told him about six months ago MI5 had approached him first to find out if he knew certain individuals and then to ask if he would work for the security service.
"He was explicit in that he refused to work for them," Nusaybah told the BBC.
It was not immediately possible to verify the claims by Nusaybah, who the BBC said was arrested immediately after giving the interview.
Scotland Yard confirmed a 31-year-old man had been arrested Friday night on suspicion of terrorism offences, but added the arrest was not directly related to Rigby's murder.
Nusaybah also told the BBC he was aware of Adebolajo's participation in activities by the banned extremist group al-Muhajiroun. Adebolajo had been photographed at multiple raucous demonstrations by the group.
The group, whose name means "the Emigrants" in Arabic, captured attention shortly after Sept. 11, 2001, when it organized an event celebrating the airline hijackers who slaughtered thousands in New York, Washington and Pennsylvania.
But attending such demonstrations, investigators say, is generally not enough to put someone under surveillance or to lead authorities to believe men or women will turn violent. Trolling the Internet for extremist sites is also no proof a person will turn violent.
In last month's Boston Marathon bombings, which killed three people and injured more than 250, at least one of the suspects had been known to authorities.
Alleged Boston bomber Tamerlan Tsarnaev, killed in a shootout with police, first came to the attention of U.S. officials in early 2011, when Russia told the FBI that he and his mother were religious extremists.
-- The Associated Press