WASHINGTON — The gunman who killed 12 people at the Washington Navy Yard on Monday began his rampage by heading directly to the fourth floor, where he shot people who worked with him, and authorities are investigating whether a workplace issue sparked the killings, according to law enforcement officials and witness accounts.
People in the department where Aaron Alexis was working had concerns about his job performance, and investigators are looking into whether those concerns escalated last week, the officials said.
‘‘He was not doing a very good job, and somebody told him that there was a problem,’’ one law enforcement official said. ‘‘Our belief is that the people who were shot first were people he had issues with where he worked, people he had some sort of a dispute with. After that, it became random. . . . After the first shootings in that office, he moved around and shot people he came upon. They were then targets of opportunity.’’
Alexis, a former Navy reservist who had recent problems with mental illness, was employed by a company contracted to upgrade computers at the Navy Yard.
Workers and law enforcement officials said Alexis worked on the fourth floor, where the shootings began. Although the investigators say they do not know the exact order in which the victims were shot, they said the rampage started in an area of people who would have worked with him.
The officials cautioned that they are still trying to learn more about the severity of the dispute and whether it was an impetus for the shootings.
The law enforcement officials spoke on the condition of anonymity because the investigation is in its early stages. Three of the officials said most of the victims on the fourth floor were shot at close range and in the head.
Officials with The Experts, the subcontractor that employed Alexis, declined to comment.
FBI Director James Comey said Alexis arrived for work at about 8 a.m. Monday and parked on a deck across a narrow road from Building 197. Carrying a bag containing a Remington 870 shotgun, he entered the building and went to a bathroom on the fourth floor.
Both the stock and the barrel of the shotgun had been sawed to shorter lengths, making the weapon more compact and easier to wield, Comey said
‘‘He shot folks on the fourth floor and the third floor,’’ Comey said. Then he walked downstairs to the lobby ‘‘and shot a security guard and took a weapon from the security guard — a Beretta semiautomatic pistol — and continued moving up and down through the building, focusing on the third and fourth floors.’’