At least 13 people were confirmed dead in the Washington Yard mass shooting, including a suspect who has been identified as 34-year-old Aaron Alexis, law enforcement officials said Monday afternoon.

The deceased gunman of Monday's shooting rampage was from Texas, said Valerie Parlave, assistant director in charge of FBI's Washington Field Office, at a televised news conference.

The FBI is seeking information from the public about Alexis and has posted two of his photos online. Alexis is believed to be responsible for the shootings at the Washington Navy Yard in the southeastern area of the capital earlier Monday morning, said the FBI.

According to the FBI's description, the deceased suspect, an African American, was born in Queens in 1979, and was last known to reside in Fort Worth, Texas.

"We all know this is a horrific tragedy, certainly one of the worst in recent memory and certainly we do not remembered anything quite like this here in this city," said District of Columbia Mayor Vincent Gray at the news conference.

As for the number of wounded, local authorities have yet to confirm. But Gray said there were probably a dozen or more who have sustained injuries.

Investigators are still searching for another possible suspect who wears an olive-colored military-style uniform, Gray said.

The incident happened at 8:20 a.m. EDT (1320 GMT) when at least a shooter opened fire inside a building at the Washington Navy Yard which serves as the headquarters of the Naval Sea System Command Headquarters.

Janis Orlowski, chief medical officer of Washington Hospital Center, said three wounded people taken to the hospital were in critical but stable conditions and expected to survived.