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First responders at the Navy Yard found a scene of carnage and chaos, as police searched for as many as three gunmen, based on initial reports. Many of the building’s 3,000 employees evacuated, but thousands of other workers sheltered in place.

Authorities closed roads and locked down schools nearby during the hourslong search. Departing flights were halted at Ronald Reagan Washington National Airport, and the Washington Nationals postponed their game at nearby Nationals Park.

The Senate went into lockdown as well, though the House did not and Sergeant at Arms Terrance W. Gainer was forced to send out a letter publicly defending his decision.

Public officials contributed to the initial public confusion, releasing conflicting accounts of how many gunmen were on the loose. Chief Lanier said one of the two gunmen they were looking for was a white man wearing a khaki Navy uniform and a beret, and carrying a handgun. The other was a black man, about 50 years old, who was believed to be carrying a “long gun.”

Authorities later ruled out the white man as a suspect, but it was unclear Monday night whether Alexis carried out the attack alone. Police didn’t say whether Alexis was shot by law enforcement officers, or took his own life.

At a hospital where some of the victims were being treated, Secretary of the Navy Ray Mabus said the service was reeling from the mass shooting.

“The Navy family today suffered a horrific attack-and we are a family,” he said. “The civilians that work in the Navy and do the critical work that has to be done suffered just a stunning and horrific blow today.”

As worried families waited for news of loved ones, the names of the dead were not released immediately to the public. Mr. Mabus said it could take another day until everyone is accounted for.

An official list of victims was not planned to be released until 24 hours after families had been notified.

President Obama expressed dismay at “yet another mass shooting,” and said justice must be sought.

“I’ve made it clear to my team that we want the investigation to be seamless, so that local and federal authorities are working together,” he said.

But as soon as he expressed his sympathy for the victims, Mr. Obama made an awkward pivot to a highly partisan speech in which he warned that congressional Republicans were threatening an economic crisis in their ongoing budget battles.

Out of respect for the shooting victims, the White House canceled a “Musica Latina” event that was to have been held Monday night. Mr. Obama also issued a proclamation calling for flags to be flown at half-staff until Sept. 20 “as a mark of respect for the victims of the senseless acts of violence.”

Other Democrats were far more explicit in calling for a political response to the latest in a string of mass shootings over the past couple of years.

“When will enough be enough?” said Sen. Dianne Feinstein, California Democrat. “Congress must stop shirking its responsibility and resume a thoughtful debate on gun violence in this country. We must do more to stop this endless loss of life.”

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