At least 13 people are dead and several others were wounded after a gunman opened fire at the Washington Navy Yard on Monday, police said, spreading fear and chaos across the region as authorities sought to contain the panic.

The incident, in which the death toll rose almost hourly, represents the single worst loss of life in the District since an airliner plunged into the Potomac River in 1982, killing 78.

D.C. Police Chief Cathy L. Lanier and Mayor Vincent C. Gray announced the mounting number of casualties in a series of news conferences. The suspected shooter, identified by the FBI as Aaron Alexis, 34, living in Fort Worth, is among the 13 dead. Alexis was a military contractor, one official said.

But even hours after the rampage began, it was still unclear whether the shooting was the act of a lone gunman, or if other shooters were involved. Lanier initially said authorities were looking for two more potential shooters dressed in military style clothing. But shortly after she announced a detailed description of two suspects, city officials said one had been located and cleared.

Gray said no motive is known yet. He said there is no reason to believe it was an act of terrorism, though he added that he could not rule it out.

Gray said that in addition to those killed, about a dozen people were hurt. It was not clear whether those people were shot or suffered other injuries, he said.

Lanier described the other possible suspect, who has not been located, as a black man in his 40s with gray sideburns, wearing an olive-drab military-style uniform. He, and the man who was cleared, came under suspicion when they were seen on surveillance videos.

Police are asking anyone with information on the suspect to call 202-727-9099.

Alexis was armed with an assault rifle and a handgun, two law enforcement officials said. One said he also had a shotgun. One official said all the weapons have not been accounted for.

The first, sketchy details about the suspect offered few hints about what may have gone wrong.

Alexis grew up in Brooklyn with his mother, Sarah, and father, Anthony Alexis, according to his aunt Helen Weeks.

“We haven’t seen him for years,” Weeks said of her nephew in a telephone interview. “I know he was in the military. He served abroad. I think he was doing some kind of computer work.”

Alexis spent nearly four years in the Navy as a full-time reservist from May 2007 until he was discharged in January 2011, according to a summary of his personnel records released by Navy officials at the Pentagon.

The officials said they were still researching whether Alexis had been employed as a defense contractor or a civilian employee of the Navy, and were uncertain if he was assigned to work at the Navy Yard.

He achieved his final rank of Aviation Electrician’s Mate 3rd Class in December 2009. Officials said they did not immediately know the reasons for his discharge.

The carnage began around 8 a.m. when the U.S. Navy said that three shots were fired at Building 197, headquarters of the Naval Sea Systems Command. About 3,000 people work in the building. As the noise that some thought sounded like construction work continued, the realization set in that a gunman was firing on them.

Rick Mason, a program management analyst who is a civilian with the U.S. Navy, told the Associated Press that a gunman was shooting from a fourth floor overlook in the hallway outside his office. He said the gunman was aiming at people in the building’s first floor cafeteria.

Terry Durham said that as she and co-workers were evacuating, she saw a man down the hall raise a rifle and fire toward them, hitting a wall.

“He didn’t say a word,” said her co-worker, Todd Brundage.

One man who said he was at his desk on the second floor when the shooting began recalled hearing a loud noise “like someone dropping an old metal desk.” The man, who declined to give his name, said there was a pause, then several noises close together and he realized the danger: “There’s a shooter in the building. I started walking toward the door and I heard people running down the hall.”

Employees described the chaos, as a fire alarm sounded and people shouted, “Where is he? Where is he?”

Gregory Dade, a Navy contractor, said he and a co-worker locked themselves in a second floor office of Building 197 as soon as the shooting went on, in fits and stops. Dade, called it “terrifying.”

He heard a woman scream, glass crashing and a series of gun shots. Then he heard shouting: “Get down! Get down! This is the police.”

Finally, about 11 a.m., he and a co-worker made a break for it.

At an exit, they noticed a trail of blood running to the next building.

Tim Jirus, a Navy commander who works in Building 197, said that as he was evacuating the building he saw a co-worker who had been shot getting into a police car, and heard more shots fired from inside his workplace.

A far worse sight lay just ahead. Jirus went to an alley where he thought he would be safe, and talked briefly with a man there about what was going on. Jirus said he heard two gunshots, loudly echoing off the building. It caused him to spin around to look for the source of the noise. But when he turned back, he looked down and saw the man he had been conversing with lying on the ground, shot in the head.

Uncertain where the shooter was, he ran.

“I was just lucky,” he said. “The other person was shorter than me. There were two shots, he got that guy, he didn’t get me. . . . The randomness of it — standing right next to me, one person gets shot.”

At least two police officers were among those shot. Police on the scene said one is a D.C. police officer who was shot twice in the leg and was evacuated to a hospital. Lanier said he was in stable condition after engaging the shooter with gunfire. The other was a base officer. The D.C. officer, was conscious at MedStar Washington Hospital Center and his chances for survival were good, hospital officials said.

Janis Orlowski, the chief medical officer at Washington Hospital Center, said three victims in all were brought to the center, all in critical condition but alert, responsive and able to talk with doctors. The victims were also able to speak briefly to law enforcement officers before undergoing surgery or treatment, she said.

The other two victims at the hospital were female civilians, Orlowski said at a news conference. All are likely to survive.

The impact of the incident rippled across town, forces schools, offices and homes into an enforced lockdown.

Senate buildings on Capitol Hill went on lockdown at about 3 p.m., with no one allowed to enter or leave the building, though it was partially lifted toward the end of the day to allow staffers to go home.

The Nationals, whose ballpark is near the Navy Yard, postponed a Monday night game. Instead, they will play a double header Tuesday at 1 p.m. and 7 p.m.

Ten public and charter schools and a public school administration building in the District went on lockdown, and flights out of Reagan National Airport were briefly halted, causing delays even after they began departing again.

Adm. Jonathan W. Greenert, the chief of Naval Operations, was evacuated from his residence at the Navy Yard complex shortly after the first report of shots fired, Navy officials said.

Greenert, a four-star admiral and member of the Joint Chiefs of Staff, was safely evacuated to the Pentagon along with his wife, Darleen, said Cmdr. Ryan Perry, a Navy spokesman.

Police closed the 11th Street Bridge as well as M Street SE between Second and Fourth streets SE because of the shooting. Entrances to the Navy Yard Metro station remain open.

As helicopters circled overhead and emergency vehicles continued to rush to the scene, crowds of onlookers gathered on sidewalks and at a construction site near the Navy Yard, but police pushed them back, yelling at them to keep a distance from the grounds.

President Obama on Monday expressed sympathy for the victims of the 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.

As a long day drew to a close, weary and stunned Navy personnel filtered into Capitol Hill in their tan uniforms, blue camouflage fatigues and civilian clothes, many of them saying little, just wanting to get home after a day spent running or hiding, and then waiting.

Tom Dick-Peddie, 46, was at work in another building in the Navy Yard and said they were on lockdown for about an hour and a half, then they were evacuated to another building. Shortly after 3 p.m. they were allowed to leave.

Kate O’Neill of Arlington and Stephanie Cates-Harman of Columbia, who work in the office of the general counsel, said their office had been ordered to shelter in place until 2 p.m. While some people were very upset, and others were calm, and tried to contact relatives to assure them that they were okay.

“Texts were going fast and furious,” O’Neill said.

9月16日，位于美国首都华盛顿的海军海洋系统司令部发生一起恶性枪击案，造成至少13人死亡、十多人受伤。这是自1982年致死78人的波托马克河空难以来，华盛顿地区发生的死亡人数最多的单起事件。目前，唯一嫌犯的身份被确定为海军退伍军人阿龙·亚历克西斯，他已在现场被击毙。

***目击者讲述案发经过

当天早晨8点左右，海军海洋系统司令部总部传来三声枪响，当时大约有3000人在总部大楼里工作。起初听见异响时，部分工作人员以为是有建筑工程正在施工，然而事实上，一名凶徒将枪口对准了他们。

里克·梅森是一名项目管理分析师，他碰巧目睹了案发的经过，“我的办公室在四楼，我看见一名枪手站在办公室外的走廊里向下张望。然后，他拿起枪向一楼咖啡厅的人们扫射”。特里·德拉姆说，她和同事从办公楼撤离时，她看见一名男子站在大厅里、举起一把枪向他们开火，子弹射到了墙壁上。“他一个字也没说，”德拉姆的同事托德·布伦戴奇在一旁补充道。

另一名在二楼办公的工作人员回忆称，枪击开始时他听见了一声巨响，“就像有人丢下了一张旧金属桌”。“短暂沉默后，又有几声连续的噪音传来，这时我才意识到危险了：大楼里有一名枪手！我朝门外跑去，走廊里响起杂乱的脚步声和人们的惊呼声‘他（枪手）在哪儿？他在哪儿？’”

海军承包商格雷戈里·戴德说，听见枪声后，他和一名同事立刻把自己锁在二楼办公室里。“当时太吓人了，”戴德说，他听见女人的尖叫声、玻璃碎裂的声音和一连串的枪响，“然后有人高喊‘趴下！趴下！我们是警察”。终于，噩梦在11点左右结束了，戴德和同事走出总部大楼时，看见有血迹一直通向隔壁的建筑。

而另一名海军承包商蒂姆·吉瑞斯经历了更加惊险的一幕。听见枪响后，吉瑞斯迅速跑进一条走廊，他以为自己安全了，就同旁边的一名男子聊起当下的状况。“我听见了两声枪响，于是自然而然地四处张望寻找声音的来源。突然，我看见地上躺了一个人，就是刚才和我说话的那个人，他的头部中枪了、鲜血不断涌出，”吉瑞斯回忆说，“我不知道枪手在哪儿，我拔腿就跑。我只是走运，凶手开了两枪，他打中那个男人、却没能打中我。”

***嫌犯身份确定

目前，嫌犯的身份已被美国联邦调查局（FBI）确认为阿龙·亚历克西斯。亚历克西斯今年34岁，他随父母在纽约市布鲁克林区长大，生前居住在得克萨斯州沃斯堡市。“我们好几年没见过他了，”亚历克西斯的婶婶海伦·威克斯接受电话采访时说，“我知道他在军队，还去过国外服役，我以为他做的是与电脑相关的工作。”

自2007年5月起，亚历克西斯曾在美国海军服役近4年，直至2011年1月退役。美国官员称，他们仍在调查亚力克西斯退役后是否受雇成为一名国防承包商或者海军的文职雇员，他们不确定亚历克西斯是否被安排到海军海洋系统司令部工作。

据两名执法人员透露，行凶时亚历克西斯手持一支冲锋枪和一把手枪。也有人说，凶徒还有一把散弹猎枪。不过，上述说法均尚未被证实。

***枪击案是一人所为

枪击案发生后，华盛顿当局举行了一系列新闻发布会，对外公布案件进展。

当地时间16日，华盛顿警方负责人凯茜·拉尼尔宣布，此次恶性枪击案是“一人所为”，不存在其他嫌犯。此前，拉尼尔曾宣称警方正在搜索两名潜在嫌犯，她还公布了这两名潜在嫌犯的外貌特征。拉尼尔还表示，之前向海军海洋系统司令部的工作人员以及司令部周边民众发布的“就地避难”指令已被解除。

华盛顿市长文森特·格雷在新闻发布会上公开了部分遇难者的身份信息。据悉，遇难者的年龄从46岁至73岁不等，有关部门正在向遇难者家属告知这一不幸的消息。格雷称，目前还不清楚枪手的行凶动机，“但没有理由认为这是一起恐怖主义行为”，不过他补充道也不能排除这种可能性。

***奥巴马厉声谴责

此次枪击案已对华盛顿造成巨大影响。距离事发地点仅6分钟车程的国会参议院在16日下午3时关闭，不允许任何人进入或者离开。不过当天晚些时候，参议院的大门重新开放，议员们获准回家。备受关注的本地棒球比赛宣布推后一天举行，将在17日连赛两场。

除此之外，还有10所公立和特许学校以及一栋公立学校行政楼被封锁。在华盛顿里根国家机场，飞机被暂时禁止飞离，导致机场出现航班滞留晚点的现象。

16日，美国总统奥巴马严辞谴责美海军海洋系统司令部枪击案是“懦夫行为”，发誓将展开全面调查，把凶手绳之以法。奥巴马称，枪击案发生后，他一直在听取情况汇报，“我已发出明确指示，要求联邦和地方当局密切合作，展开无缝调查，竭尽全力将制造这起懦夫行为的人绳之以法”。