President Obama spoke at a memorial service Sunday to honor the 12 victims of Monday's Navy Yard shootings.
"The tragedy and the pain that brings us here today is extraordinary. It is unique," he said.
But Obama also noted Monday's incident is the fifth mass shooting he has witnessed as president. "Once more, our hearts are broken," he said.
The president said he worried that there is a resignation that these types of tragedies are bound to happen.
"We must insist here today: There is nothing normal about innocent men and women being gunned down where they work," he said.
He called on the nation to address gun violence, saying that resignation sometimes comes from a sense of political stagnation.
"By now, though, it should be clear that the change we need will not come from Washington, even when tragedy strikes Washington. Change will come the only way it ever has come, and that's from the American people," he said.
Before the Sunday afternoon service, the president and first lady Michelle Obama met with families who lost loved ones in Monday's attack. The violence took place less than three blocks away from the Marine barracks in southeast Washington, D.C.
The private memorial was held on the facility's parade grounds, where some 4,000 people attended. Speakers recalled the sense of duty and purpose of the victims, who ranged in age from 46 to 73.
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Early reports from the scene describe a mix of uniformed officers and civilians. Those in attendance included Joint Chiefs of Staff Chairman Gen. Martin Dempsey, Sen. Barbara Mikulski, D-Md., Rep. Steny Hoyer, D-Md., and Attorney General Eric Holder.
Earlier Sunday, gun rights activists took to the morning talk shows to speak against making new attempts to curb Americans' rights to own guns.
The head of the National Rifle Association, Wayne LaPierre, said that more personnel at military facilities should be armed to try to stop such attacks.
Speaking on NBC's Meet the Press today, LaPierre said that in the case of the Navy Yard shooting, the problem was that too many people at the facility were unarmed.
"All these brave men and women that are trained in firearms, that signed up to serve in the military, they're largely disarmed on our military bases," LaPierre said. He recommended policy changes that would keep more military personnel armed.
"The problem is, there weren't enough good guys with guns," he said. "When the good guys with guns got there, it stopped."
LaPierre has made similar arguments in the past, including after the shootings last December at Sandy Hook Elementary in Newtown, Conn.
That tragedy, in which school children lost their lives, prompted a push by the president and his allies to change America's gun control laws. But legislation that sprang from the effort failed to get out of the Democratically controlled Senate.
NPR's Liz Halloran noted the uncertainty over the nation's gun laws in a post earlier this week:
"The massacre of 20 school children and six adult school employees at Sandy Hook Elementary School in Connecticut last December took the nation to a place of horror that it had never visited.
"Yet little has changed since, leaving the political component of the debate in an uneasy state."
Update at 5:27 p.m. ET. President Obama Speaks:
The president made a point of saying this shooting was not "routine," but that it "echoes other recent tragedies." He noted that he has grieved with five communities at mass shootings as president, all of which "occur against a backdrop" of daily shootings.
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