Authorities in Washington D.C. said Monday night they were confident that slain suspect Aaron Alexis had no help in a mass shooting at the Washington Navy Yard, the Washington Post reported.

"We are comfortable we have the single and sole person responsible for the loss of life on the base today," Police Chief Cathy Lanier said at a press conference Monday night, explaining that her department had gone through "all means that we have available" investigating the possibility of a second shooter in the attack. Alexis was killed by security after allegedly killing 12 people during the shooting.

District of Columbia Mayor Vincent Gray said at the press conference that the families of seven of the victims have been notified. Gray did not reveal the victims' names, but did say they ranged between 46 and 73 years of age. Another eight people were injured in the shooting, including a local police officer, Scott Williams. Gray said Williams was recovering and "in very good spirits."

WUSA-TV reported that a spokesperson with tech company Hewlett-Packard, Michael Thacker, confirmed that Alexis worked for a subcontractor for the company.

"We are deeply saddened by today's tragic events at the Washington Navy Yard," Thacker told WUSA. "Our thoughts and sympathies are with all those who have been affected."

Watch the press conference, posted on Monday night by WUSA, below.