WASHINGTON -

President Barack Obama honored police officers killed in the line of duty in the past year, telling a crowd of thousands that members of law enforcement never let down their guard and that the public they serve should "never let slide our gratitude."

Obama spoke Wednesday at the National Peace Officers Memorial Service on the steps of the U.S. Capitol building.

"The brave officers that we gather for today devoted themselves so fully to others -- to serve and to protect others -- that in the process they were willing to give their lives," Obama said. "So today, let us not remember them for how they died, but also for how they lived."

IMAGES:  Photos from memorial service, local families in attendance
UNCUT VIDEO: Thomas family greets president

Also in the crowd were the families and fellow officers of Clay County Detective David White and Florida Corrections Officer Ruben Thomas of Lake City, who were both killed in 2012.

Those two were among the 143 names of officers who died that were read aloud during the Roll Call of Heroes. After the ceremony, their family members pinned flowers to a wreath.

"I think our family was in complete awe from all of the honor that we received from all the different law enforcement agencies across the country and even some from other countries such as England," said Ruben Thomas' mother, Paula.

In his remarks, Obama said he is always mindful of the sacrifices made by police officers assigned to protect him and his family.

"We can never repay our debt to these officers and their families, but we must do what we can with all that we have to live our lives in a way that pays tribute to their memory," Obama said of the fallen officers.

David White's widow and Ruben Thomas' sister were among those who added flowers to a wreath, which will be watched over by honor guards of police agencies and sheriff's office from around the nation, including a team from the Clay County Sheriff's Office, who have been there before.

"We like to line the team up and get a picture with the Capitol in the backdrop, and the last time we did that was with David," said Sgt. Mark Tate.