LONDON -- Prince William, the Duchess of Cambridge and Prince Harry joined first responders and people injured in the March 22 terror attack in London for a multi-faith service Wednesday meant to underscore the nation's resolve to remain united despite adversity.

Some 1,800 people took part in the service at Westminster Abbey, just a few hundred meters (yards) from where Khalid Masood mowed down pedestrians before fatally stabbing a police officer outside Parliament. Prayers were offered by representatives of the Muslim, Jewish, Sikh and Christian communities.

"What happened a fortnight ago leaves us bewildered," Dean of Westminster John Hall said. "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 possibly know, against whom he had no personal grudge, no reason to hate them and then run at the gates of the Palace of Westminster to cause another death? It seems likely that we shall never know."

Police, ambulance workers and firefighters who helped the dying and injured stood shoulder-to-shoulder with victims and their relatives during the service.

Injured tourist Melissa Cochran arrived at the abbey in a wheelchair. Her husband, Kurt, 54, died in the attack. The couple, from the U.S. state of Utah, had been celebrating their 25th wedding anniversary when they were caught up in the rampage.

The other people killed were police constable Keith Palmer, retired window cleaner Leslie Rhodes, 75, and school administrator Aysha Frade, 44.

Police shot the 52-year-old Masood dead after he stabbed Palmer in a Parliament courtyard.

Before the service, Prince William stopped briefly to lay a wreath of red and white spring flowers at the abbey's Innocent Victims Memorial, a slate circle that remembers those who have suffered death, torture and oppression throughout the world.

The royal trio's presence underscored the ceremony's purpose of inspiring hope and looking to the future.

Prince William, his wife, Kate, and Prince Harry met in private with the victims.

The royals also met with acting U.S. Ambassador Lewis Lukens, Romanian Ambassador Dan Mihalache and with people who responded to the accident.

"He talked to us about occupational health, mental health and sort of making sure that we look after ourselves with regard to counselling," James Southgate, a police sergeant based in Lambeth, said. "And just thanking us for our hard work and arriving so promptly."