Still, counter-terrorism 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 US have similarly pledged to review their procedures in the wake of the Boston Marathon bombing, with the Boston police commissioner saying that 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 counter-terrorism 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 on Thursday that 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 counter-terrorism 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 that 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 that a 31-year-old man had been arrested Friday night on suspicion of terrorism offenses, but added that 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 to violence.
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 US officials in early 2011, when Russia told the FBI that he and his mother were religious extremists.
The FBI investigated them, and their names were added to a Homeland Security Department database used to help screen people entering and leaving the US.
But the FBI found nothing linking them to religious extremists or terrorists, and asked the Russians twice for more information. The FBI never heard back and closed its investigation in June 2011.
In the fall of that year, the Russians reached out to the CIA with the same concerns.
The CIA shared this with the FBI, and also asked that the names of Tamerlan and his mother, Zubeidat Tsarnaeva, be entered into a massive government database of people with suspected terrorist ties.
The FBI again reached out to Russia for more information, and never heard back. Officials in Boston have said the FBI did not initially share the warnings with them, though they acknowledge they might not have uncovered or disrupted the plot based on those warnings.
Boston Police Commissioner Ed Davis announced last week that the Boston Police Department and mayor's office will conduct reviews of the response to the bombings. Davis told Congress that cities should look at deploying more undercover officers and installing more surveillance cameras - but not at the expense of civil liberties.
"I do not endorse actions that move Boston and our nation into a police state mentality, with surveillance cameras attached to every light pole in the city," Davis said.
Britain is already one of the most heavily watched countries in the world with more than 4.3 million CCTV cameras around the country.
Europol Director Rob Wainwright cautioned against changes that could disturb the balance struck by the "principle of proportionality."
"Investigators have to prioritize their work," Wainwright told The Associated Press. "There are limited resources, but it's not just a question of resources."
Technological advances, for instance, help investigators but also have led to a deluge of data for them to sift through, he said, noting, "The more data you have, the more potential you have for suspects."
Britain, meanwhile, was bracing for potential clashes with right-wing activists, who have promised demonstrations, as well as possible copycat terror attacks in the wake of Wednesday's killing.
Some 1,200 extra police have been put on alert in London.
Wednesday's attack was captured on video by passersby and made for gruesome viewing - one man is seen with his hands stained red with blood and holding two butcher's knives as he angrily complained about the British government and troops in foreign lands.
Analysts say the attackers wanted the publicity to inspire copycats. Already, there has been increased chatter on militant sites, they said.
"We can see the tempo being raised," said Maajid Nawaz, a former jihadist who is now with the London-based anti-extremist Quilliam Foundation. "One of the reasons why these guys acted in this theatrical way was because of the propaganda effect so others would be inspired to do the same thing."
A British government official, who spoke on condition of anonymity because he wasn't authorized to speak about the ongoing investigation, confirmed the increase in chatter since Wednesday's attack but said no specific or credible plots had been detected.
Britain's terror threat level has remained unchanged at "substantial" - the middle of five possible rankings - since the slaying of Rigby.
His anguished widow, Rebecca Rigby, spoke of her loss Friday at a news conference at his unit's headquarters. "I love Lee and always will," she said, sobbing.
His stepfather, Ian Rigby, read a statement on the family's behalf, including the final text the soldier had sent to his mother, who was too upset to speak.
"The last text he sent to his mum read, 'Goodnight mum, I hope you had a fantastic day today because you are the most fantastic and one in a million mum that anyone could ever wish for. Thank you for supporting me all these years, you're not just my mum, you're my best friend. So goodnight, love you loads," Ian Rigby said.