LONDON (Reuters) - Police investigating the murder of a soldier hacked to death on a busy London street were looking on Friday into whether the two suspected killers, British men of Nigerian descent, were part of a wider conspiracy.

The two suspects, aged 22 and 28, are under guard in hospitals after being shot and arrested by police following the murder of 25-year-old Afghan war veteran Lee Rigby on Wednesday in broad daylight. They have not yet been charged.

Detectives were also questioning another man and a woman, arrested on Thursday on suspicion of conspiracy to murder, as they tried to determine whether those responsible had links to militants in Britain or overseas.

“This is a large, complex and fast-moving investigation which continues to develop,” police said in a statement.

“Many lines of inquiry are being followed by detectives, and the investigation is progressing well.”

One of the assailants, filmed justifying the killing as he stood near the body holding a knife and meat cleaver in bloodied hands, was named by acquaintances as 28-year-old Londoner Michael Adebolajo, a British-born convert to Islam.

Little is known so far about the other man.

The murder, just a month after the Boston Marathon bombing and the first Islamist killing in Britain since local suicide bombers killed 52 people in London in 2005, revived fears of “lone wolves” who may have had no direct contact with al Qaeda.

Police chiefs said they would have 1,200 extra officers on the streets in London overnight and at key locations such as religious venues and transport hubs.

“It will be assessed on a rolling basis depending on the picture. I’m sure there will be heightened numbers for a little while to come,” a spokesman said.

A source close to the investigation told Reuters the attackers were known to Britain’s MI5 internal security service, raising questions about whether it could have been prevented. Adebolajo had handed out radical Islamist pamphlets, but neither was considered a serious threat, a government source said.

Another source close to the inquiry said the local backgrounds of the suspects in a multicultural metropolis - nearly 40 percent of Londoners were born abroad - and the simplicity of the attack made prevention difficult.

“Apart from being horribly barbaric, this was relatively straightforward to carry out,” the source said. “This was quite low-tech, and that is frankly pretty challenging.”

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Anjem Choudary, one of Britain’s most recognized Islamist leaders, told Reuters Adebolajo was known to fellow Muslims as Mujahid - a name meaning ‘fighter’: “He used to attend a few demonstrations and activities that we used to have in the past.”

He added that he had not seen him for about two years: “He was peaceful, unassuming, and I don’t think there’s any reason to think he would do anything violent.”

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POLICE LOOKING FOR LINKS

The two men used a car to run down Drummer Rigby outside Woolwich Barracks in southeast London and then attacked him with a meat cleaver and knives, witnesses said.

The pair told shocked bystanders they acted in revenge for British wars in Muslim countries.

“We swear by almighty Allah we will never stop fighting you. The only reason we have done this is because Muslims are dying every day,” Adebolajo was filmed saying by an onlooker. “This British soldier is an eye for an eye, a tooth for a tooth.”

Rigby, who had a two-year-old son, was not in uniform. The bandsman was working locally as an army recruiter.

“All he wanted to do from when he was a little boy was to be in the army,” his family said in a statement. “He wanted to live life and enjoy himself.”

In Nigeria, with a mixed Christian-Muslim population and where the authorities are battling an Islamist insurgency, a government source said there was no evidence the Woolwich suspects were linked to groups in west Africa.

British investigators are looking at information that at least one of the suspects may have had an interest in joining Somalia-based Islamist rebel group al Shabaab, which is allied to al Qaeda, a source with knowledge of the matter said.

Al Shabaab linked the attack to the Boston bombing and last year’s gun attacks in the southern French city of Toulouse.

“Toulouse, Boston, Woolwich ... Where next? You just have to grin and bear it, it’s inevitable. A case of the chickens coming home to roost!” the rebels said on Twitter.

Peter Clarke, who led the investigation into the 2005 bombings, popularly known as 7/7, said that if the Woolwich attackers did turn out to be acting alone, it showed the difficulty the security services faced in trying to stop them.

“An attack like this doesn’t need sophisticated fund raising and sophisticated communications or planning,” he told Reuters. “It can be organized and then actually delivered in a moment.”