(CNN) - A maintenance worker who tried to warn others was among those shot Monday morning at the Washington Navy Yard.
U.S. Navy Cmdr. Tim Jirus told CNN he was standing in an alley at the bottom of a fire escape supervising the evacuation when a worker from a nearby maintenance building approached him.
"He walked up and told me that he heard that there was a shooter in our building," Jirus said. "And we were just standing there maybe three feet away having a conversation, and then we heard two more gunshots, and he went down and that's when I ran."
Jirus said he did not know the man. He was "fairly certain he is dead, because he was shot in the head."
"I don't feel lucky that he got hit instead of me, but I feel lucky to be here," Jirus said.
District of Columbia Mayor Vincent Gray said that "it appears we have at least 12 fatalities" but did not provide any specifics on those killed.
At least one person, and possibly one other, opened fire in Building 197, which houses the headquarters for the Naval Sea Systems Command, Gray said. Police said one suspected shooter was dead, but it's unclear if that suspect was among the 12 fatalities.
The deaths include a man "in his 60s" who suffered a wound to his head. He was pronounced dead on arrival at George Washington University Hospital, a doctor said.
Dr. Babak Sarani said the man suffered a gunshot wound to his left temple.
Paramedics were unable to keep the man alive on the way to the hospital, said Sarani, chief of trauma and acute care at George Washington University Hospital. "Unfortunately, the injury was not survivable."
Three other shooting victims who were flown to MedStar Washington Hospital Center are expected to survive, hospital Chief Medical Officer Janis Orlowski told reporters at a news conference about three hours after the attack began around 8:20 a.m. Monday.
There were "individuals at the scene who will not be transported because they are deceased," Orlowski said.
A Navy spokesman said there were at least 10 people wounded in the shooting at Naval Sea Systems Command headquarters.
The shooting victims include a Washington Metropolitan police officer, said Metropolitan Police Chief Cathy Lanier.
The three victims flown by helicopter to the MedStar Washington Hospital Center were "alert" and "responsive," Orlowski said. "They do have severe injuries, but we've been able to speak with all of them. They've not been able to give us any information about what happened to them."
The wounded police officer -- a male -- was in surgery for gunshot wounds that "involve bones and blood vessels of lower legs," Orlowski said.
The other victims at MedStar Washington Hospital Center were women, including one shot in the shoulder and the other with a head wound, Orlowski said. A helicopter plucked one of the wounded women from a roof and carried her to the hospital, she said.
"Their chances of survival are very good," she said.
Doctors have been told to expect more victims, she said.
Relatives of people who worked in Building 197 gathered near the Navy yard, hoping for information about their loved ones.
Jacqueline Alston said she has not heard from her husband, Ernest, who is a custodial worker there.
"Right now, I'm asking God to let me hear that voice, to let me see that man again," Alston said. "All I know is, I'm supposed to be patient, which I am trying to hold on, being patient and understanding, and ask God what created this problem? What started this?"