THE British police are investigating the reason behind Wednesday's terror attack in central London that left four people dead and at least 50 injured.

Khalid Masood, 52, drove a rented SUV across the crowded Westminster Bridge, leaving a trail of dead and wounded. Then he jumped out and attacked Constable Keith Palmer, an officer guarding Parliament, stabbing him to death before being shot to death by police.

Although the Islamic State group claimed responsibility for the attack, police are investigating whether the homegrown killer acted totally alone inspired by terrorist propaganda, or if others have encouraged, supported or directed him, according to Britain's top counter-terrorism officer Mark Rowley.

British police said Friday they had arrested two more suspects related to the London attack, the deadliest in Britain since the 2005 London suicide bombings that killed 52 people.

Rowley described the overnight arrests of two men aged 27 and 35 in Birmingham and Manchester as "significant."

At least 10 people have been arrested over the attack, all on suspicion of preparation of terrorist attacks.

British police say six people, including two women and four men, being held in connection with the attack have been released without charge. They were all arrested in Birmingham, central England, where Masood recently lived.

Masood, born into a middle-class family in Kent, southeastern England, moved through several addresses in England.

British newspaper the Daily Mail said he was brought up by his single mother in the seaside town of Rye, East Sussex, southeastern England, later converted to Islam and changed his name.

Former schoolmates told the Daily Mail that he had been a popular student who excelled academically and at sport, and that he was a "happy-go-lucky" character.

He later began drinking and using drugs and slipped into criminal behavior, receiving convictions for assault and possession of offensive weapons between 1983 and 2003, according to the paper.

British Prime Minister Theresa May said Masood was investigated in relation to concerns about violent extremism years ago. But she called him "a peripheral figure."

"Our working assumption is that he was inspired by international terrorism," Rowley said.

Masood's age does not fit the profile of militant attackers, who are typically younger than 30, according to counter-terrorism officers.

"Masood is not atypical in being a British-born convert with a criminal record. He was slightly more unusual in being older, but we do not know how long ago he was radicalized," Shashank Joshi, a senior research fellow at London's Royal United Services Institute, told Reuters.

"If it was in prison, this would be a common pathway," Joshi said.

The Sun tabloid said he married a Muslim woman in 2004 and moved the following year to Saudi Arabia. Investigators were trying to determine how long he stayed and what he was doing in the kingdom.

The Saudi embassy in London on Friday issued a statement saying Masood was not known to their security services and did not have a criminal record in Saudi Arabia.

In one of the last places Masood lived, a home in Birmingham, neighbors recalled him as a quiet man.

Birmingham is home to many British Muslims. There are over 213,000 Muslims in the city, more than one-fifth of the population, according to the 2011 census.

Sabeur Toumi, manager of the hotel in the beachside city of Brighton on the south coast of England where Masood stayed the night before the attack, said he seemed unusually outgoing and mentioned details about his family, including having a sick father.

"He was normal, in fact friendly, because we spent possibly five or 10 minutes talking to him about his background and where he came from," Toumi told Sky News. He was "laughing and joking, telling us stories about where he lived."