LONDON (Reuters) - Queen Elizabeth’s grandsons Princes William and Harry joined relatives of four people killed in a deadly attack near the London parliament two weeks ago for a “Service of Hope” on Wednesday.

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Khalid Masood, 52, plowed a rented car into pedestrians on Westminster Bridge killing three people and injuring dozens more before running through the gates of parliament and stabbing a policeman to death. He was then shot dead.

British police have said it was a terrorist attack but that Masood appeared to have been acting alone. All other suspects arrested in connection with the incident have now been released.

Families of the victims: U.S. tourist Kurt Cochran, 54, college worker Aysha Frade, 44, retired window cleaner Leslie Rhodes, 75 and Pc Keith Palmer, 48, joined Harry, William and his wife Kate for the service at Westminster Abbey, a stone’s throw from the scene of the attack a fortnight ago.

“The violent assault two weeks ago against Londoners and visitors to this city from around the world and the killing of a police constable on duty at the Palace of Westminster have shocked people everywhere,” said John Hall, the Dean of Westminster.

“Those killed and injured included Londoners but also people from the United States of America, from Romania, France, South Korea, Italy, China, Australia, Greece, Portugal, Germany, Poland and Ireland.”

The congregation included those injured in the attack along with witnesses, those from the emergency services who were first on the scene and representatives from major religions.

William and British Home Secretary (interior minister) Amber Rudd gave the readings while London Mayor Sadiq Khan delivered one of the prayers.

Investigators believe Masood, a former violent criminal who had converted to Islam, had become self-radicalized by accessing extremist material on the internet. They said they had found nothing to link him to radical groups at home or militants abroad.

“What could possibly motivate a man to hire a car and take it from Birmingham to Brighton to London and then drive it fast at people he had never met, couldn’t possible know, against who he had no personal grudge ... and then run at the gates of the Palace of Westminster to cause another death,?” Hall said.

“It seems likely that we shall never know. What on earth did he hope to achieve?”