WASHINGTON - In a somber ceremony honoring police bravery, President Obama on Monday awarded the Medal of Valor posthumously to Philadelphia Police Sgt. Robert Wilson III, killed last year as he tried to buy a gift for his son - and wound up disrupting an armed robbery.

Tears streamed down the face of Wilson's grandmother Constance Wilson as she accepted the medal from the president in the White House's East Room.

"Please know how deeply sorry we are for your loss, how grateful we are for Sgt. Wilson's service," Obama told her as well as Wilson's sister, Shakira Burroughs, and brother, Dareon, in his opening remarks.

A dozen other officers were also honored for acts of bravery since 2013.

"The public safety officers we recognize today with the Medal of Valor found courage not in search of recognition, they did it instinctively," the president said. "This is an award that none of them sought."

Wilson, a father of two, was the only one to lose his life.

He was gunned down in a North Philadelphia video-game store on March 5, 2015. He had stopped by the GameStop on Lehigh Avenue to buy a birthday present for his son, who was turning 10, and do a security check.

Wilson was at the counter when two brothers walked in, announced a robbery, and brandished semiautomatic handguns.

The eight-year member of the force drew his gun - and a shootout erupted.

Police said Wilson fought to the end, firing back even after being hit, until a bullet struck him in the head. They credited him with drawing attention away from other customers and store workers. The brothers were later arrested and are awaiting trial on murder and other charges.

The medal, created by Congress in 2001, honors public safety officers who "have exhibited exceptional courage, regardless of personal safety."

In a room draped with gold curtains and marked by portraits of George and Martha Washington, the recipients Monday came to the stage in turn. Obama placed the medal around each of their necks, and greeted them with a pat on the shoulder.

One, Donald Thompson, an officer in Los Angeles, was off duty when he pulled a man from a burning car, sustaining burns during the rescue. Another honoree was an Oklahoma officer who rescued a 2-year-old from a man holding the child at knifepoint. Several had put themselves in harm's way to stop shootings.

Wilson, who came from a family of police officers, was the first Philadelphian honored by the award. Other recipients at the ceremony thanked his family for his service.

"We are a brotherhood - a brother- and sisterhood, I guess you could say," said Jason Salas, a Santa Monica, Calif., officer recognized for his role in stopping a shooting rampage at a local college.

Even when an officer as far away as Philadelphia is killed, he said, "it ripples all the way to the West Coast with us. Any time an officer falls, it definitely hurts."

Thompson, the Los Angeles officer, said two of his academy classmates had been killed in the line of duty, and estimated that he knew more than a dozen others who have fallen. "It hurts so much - just to see the grieving family, it breaks your heart," he said.

Obama also thanked the families, noting the long nights waiting for loved ones to come home. "We know it never gets easier, and we thank you for that. And of course, we honor those who didn't come home," he said.

In a brief interview after the ceremony, Wilson's grandmother said she still has "a big hole" in her heart.

"It never leaves my mind," she said. "I think of Robbie every day."

When she came to the stage to accept her grandson's medal, Obama bowed his head and wrapped an arm around her.

As she stepped down, the applause in the room rang a little louder than before, and a little longer.