(NBC) - President Barack Obama and the first lady will join families of the victims of Monday's deadly Navy Yard shooting at a memorial service in the nation's capital on Sunday.

The service is scheduled for 5:00 p.m. at the Washington Marine Barracks, just blocks from where authorities say Aaron Alexis shot and killed 12 navy yard workers on Monday afternoon and injured eight others.

D.C. Mayor Vincent Gray, Navy Secretary Ray Mabus and Defense Secretary Chuck Hagel are all scheduled to attend. The president is expected to speak at 5:30 p.m.

The Washington Navy Yard was reopened Thursday, even before flags returned from their positions at half-staff at sunset on Friday, as ordered by the president after the shooting.

Alexis, 34, former Navy reservist who had been working as a civilian contractor with access to the site, was killed by officers who responded the shooting spree.

In the midst of the outbreak on Monday, President Obama called the massacre a "cowardly act" and sent his condolences to the families of the victims.

The president and the first lady will visit with the families of the victims — fathers and mothers, Navy veterans and civilians, who ranged in age from 46 to 73.

"The men and women who lost their lives devoted their careers to protecting our country," Michelle Obama said on Wednesday, adding, "we hold all of their loved ones in our hearts at this very difficult time."

The service is not open to the public, but some 4,000 people have been invited, barracks spokesman Capt. Jack Norton told The Associated Press.

"The president will want to mourn the loss of these innocent victims and share in the nation's pain in the aftermath of another senseless mass shooting," White House press secretary Jay Carney said Wednesday, repeating the terminology President Obama two days earlier when he said, "So we are confronting yet another mass shooting."

His statement mirrored that of one he gave just nine months earlier, after 20 children and six adults were shot and killed at a school in Newtown Connecticut.

"As a country, we have been through this too many times. Whether it's an elementary school in Newtown, or a shopping mall in Oregon, or a temple in Wisconsin, or a movie theater in Aurora," Obama said in December, listing the handful of the times during his presidency when he has stood before the nation to express his sorrow and provide comfort.

Before then, the president also offered consolation to families of victims of shootings in Binghamton, New York; Tucson, Arizona and Fort Hood, Texas — the three tragedies left 31 innocent people dead.

Two weeks after the elementary school shooting in Newtown, Obama said in an interview with NBC's David Gregory, "That was the worst day of my presidency, and it's not something that I want to see repeated."

On Saturday night, at an awards dinner for the Congressional Black Caucus Foundation, the president once again lamented some of these tragedies. He also called for action against an outbreak of violence in his home city of Chicago, which started on Thursday when 13 people, including a 3-year-old, were injured in a single shooting spree.

"As long as there are those who fight to make it as easy as possible for dangerous people to get their hands on a gun, then we've got to work as hard as possible for the sake of our children," he said at the event.

President Obama will give another address at Sunday's memorial. Secretary of Defense Chuck Hagel is also scheduled to speak.