U.S. officials said there were no immediate signs that terrorists were behind the attack at the Naval Sea Systems Command office.
A total of 13 people were killed and about a dozen others wounded by at least one gunman at the Washington Navy Yard office located on a base in southeast Washington. The Navy Yard is located less than three miles from the White House.
Washington Mayor Vincent Gray told reporters that the investigation into the attack is ongoing and it is far too soon to draw any conclusions.
“We don’t know what the motive is at this stage,” Gray said, adding that the investigation is ongoing. “We don’t have any reason at this stage to suspect terrorism, but certainly it has not been ruled out.”
One U.S. official said intelligence agencies that monitor the Internet uncovered no immediate claims of responsibility for the attack from online Islamist terrorists.
The FBI is said to be looking into whether the shooting was carried out by what are termed “homegrown violent extremists” or HVEs.
The Boston Marathon bombing is believed to have been the work of two such radicalized Muslims.
Several jihadists wrote in Arabic on Twitter shortly after the shooting began around 8:20 a.m. ET that it appeared to be motivated by Islamist extremism. Some used the hashtag “Al Qaeda” to report news of the shooting.
Some jihadists said the shooting was part of global jihad but none indicated inside knowledge of the killings, suggesting the comments are probably speculation or propaganda.
One Twitter jihadist said the attacks took place close to the 12th anniversary of the al Qaeda attacks on New York and Washington and thus appeared to be linked to Islamism.
A jihadist named Idaat Amniyah (@abdallahsaker) who claimed to be part of the Middle East group Fatah al-Islam stated that the shooting showed “we have moved from the defensive to the offensive stage” of global jihad.
Another jihadist, identified as Abu-Osama al-Muwahhid (@ab_osama1), issued a Tweet praising the attack and quoted al Qaeda leader Osama bin Laden as saying the United States would not have security because of the Israel-Palestine dispute. The posting said that the attack was the work of “not one lone wolf, but three.”
Still another jihadist commented incorrectly that news reports claimed the attackers were wearing a white garment called “al kafan” that is used as a death shroud, indicating they were on a suicide mission.
D.C. Police Chief Cathy Lanier told reporters that in addition to the one gunman found dead at the Navy Yard building, intially two other suspects thought to be wearing military garb were being sought. One of the two suspects was ruled out and police continued a manhunt for the second suspect, who was spotted on security surveillance video.