WASHINGTON - Investigators have found no evidence of a second suspect in the shooting spree at the Washington Navy Yard that killed 13 people, including suspected gunman Aaron Alexis, officials told a news conference on Monday.

"We have continued to pursue the possibility of there being another shooter but we don't have any evidence, any indication, at this stage that there was another shooter," Washington Mayor Vincent Gray said.

Washington police chief Cathy Lanier said investigators lifted the "shelter in place" for neighborhoods near the Navy Yard once they had exhausted all leads for a possible second shooter.