LONDON - 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.

Drummer Lee Rigby, of the British Army's 2nd Battalion The Royal Regiment of Fusiliers, is seen in an undated photo released on May 23, 2013. Rigby was killed May 22 in an attack by two men in Woolwich, southeast London, the Ministry of Defence said on Thursday. [Photo/Agencies]

"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-Qaida.

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."

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."