* Khalid Masood had long string of convictions
* Police suspect "Islamist-related terrorism"
* Masood was on the fringes of a previous investigation
* UK had little knowledge of Masood on eve of attack
* Spent time in Saudi Arabia but not on security radar (Adds Saudi statement, paragraph 7)
By Michael Holden
BIRMINGHAM, England, March 24 (Reuters) - Before he killed four people in Britain's deadliest attack since the 2005 London bombings, Khalid Masood was considered by intelligence officers to be a criminal who posed little serious threat.
A British-born convert to Islam, Masood had shown up on the periphery of previous terrorism investigations that brought him to the attention of Britain's MI5 spy agency.
But the 52-year-old was not under investigation when he sped across Westminster Bridge on Wednesday, mowing down pedestrians with a hired car before running into the grounds of parliament and fatally stabbing an unarmed policeman. He was shot dead by police.
"Our investigation focuses on understanding his motivation, his operation and his associates," Britain's senior counter-terrorism police officer, Mark Rowley, told reporters.
A European government source familiar with the investigation said Masood's name had cropped up about five years ago on the margins of an MI5 counter-terrorism investigation but interest in him had swiftly dissipated.
At the time of the attack, British authorities had "no intelligence about how he got to this point," the source said, adding that investigators were now rushing to piece together his background and contacts.
The Saudi Arabian embassy in London said Masood had spent time working in the kingdom but did not come to the attention of the security services there. He was there for two one-year periods, from November 2005 and April 2008, when he worked as an English teacher, and also visited briefly in March 2015.
A U.S. government source, speaking on condition of anonymity, said although Masood's associates included people suspected of being keen to travel to join jihadi groups overseas, he himself never did.
Islamic State claimed responsibility for Masood's attack, although it was unclear what links - if any - he had with the militant group. Police said there had been no prior intelligence about his intent to mount an attack.
"An act of terrorism tried to silence our democracy," Prime Minister Theresa May told parliament. "He took out his rage indiscriminately."
Born Adrian Russell Ajao in Kent, southeast of London, on Christmas Day in 1964, he moved though several addresses in England and lived recently in Birmingham, central England.
The Daily Mail newspaper said he was brought up by his single mother in the town of Rye on England's south coast, later converting to Islam and changing his name. Other media reports said he was a married father-of-three and into bodybuilding.
Police have released few details about Masood and what might have led him to carry out Wednesday's attack, the deadliest in Britain since four young British Islamists killed 52 people in the 2005 London suicide bombings.
Known by a number of aliases, he racked up a string of convictions, but none for terrorism-related offences. His occupation was unclear.
He first came to the attention of the authorities in November 1983 when he was found guilty of causing criminal damage. His last conviction came 14 years ago in December 2003 for possession of a knife.
"Our working assumption is that he was inspired by international terrorism," said Rowley of London's Metropolitan Police.
Masood's age does not fit the profile of militant attackers, who are typically younger than 30, according to counter-terrorism officers.
Shashank Joshi, Senior Research Fellow at London's Royal United Services Institute, said an MI5 report based on studies of several hundred British extremists found that half were born in Britain. Few were raised in very religious households and many, like Masood, were converts.
"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 radicalised," Joshi told Reuters.
"If it was in prison, this would be a common pathway. Given the diversity of Islamist extremists, Masood doesn't look too unusual."
Detectives are questioning 11 people in custody, having made what they said were two more significant arrests in central and northwest England.
Iwona Romek, a former neighbour from Birmingham, told reporters: "When I saw the pictures on TV and in the papers of the man who carried out the attack, I recognised him as the man who used to live next door.
"He had a young child, who I'd think was about 5 or 6 years old. There was a woman living there with him, an Asian woman. He seemed to be quite nice, he would be taking care of his garden and the weeds."
In December, she said, he suddenly moved out.
Birmingham has been home to many British Islamists. According to a study by the Henry Jackson think-tank earlier this month, 39 of 269 people convicted in Britain of terrorism offences from 1998 to 2015 came from the city.
There are over 213,000 Muslims in Birmingham, more than a fifth of the population, according to the 2011 census, and there has been growing concern about divisions in the diverse city.
Masood hired the car he used in Wednesday's attack in Birmingham from rental firm Enterprise, and he may have rented an apartment not far from the Enterprise offices, which were among properties raided by armed police officers.
On the eve of the attack, Masood spent his last night in a budget hotel in Brighton on the south coast, according to a fellow guest who recognised him.
"Nothing in his demeanour or his looks would have given me any thoughts that would make me think he was anything but normal," said Michael Petersen.
(Additional reporting by Mark Hosenball in Washington; Editing by Guy Faulconbridge, Mark Trevelyan and Peter Graff)