A District man has been sentenced to 22 years in prison for the armed robbery of his grandparents.

A D.C. jury on Wednesday sentenced Riley Mercer on charges of conspiracy, burglary while armed, kidnapping while armed, armed robbery of a senior citizen and related weapons offenses.

Mercer, 40, lived with his grandparents in Northeast Washington and worked for the security company they used for their home. During his employment, Mercer learned his grandparents kept a large amount of cash in a safe in their bedroom closet and that his grandmother was the only person with the combination.

In the early morning hours of Sept. 29, 2011, Mercer, along with two other men, all wearing masks, snuck into his grandparents' room and pointed their guns at the victims. Both are in their 70s.

One of the men led Mercer's grandmother to the safe. When he threatened to shoot the grandmother because she was not moving quickly enough for him, the victim heard Mercer's voice say, "Don't hurt her."

Although the men wore masks over their faces, the victim was certain it was her grandson's voice, prosecutors said.

The men made off with about $15,000 in cash from the safe, leaving the victims tied and bound on their bed. The victims' eldest daughter eventually heard them crying for help and untied them.

The other two masked men have never been identified.