British 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.

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.

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.

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.

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.

"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."

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," Choudary said.

He added that he had not seen him for about two years.

Reuters