Rhode Island police warned the U.S. Navy last month that Washington Navy Yard gunman, Aaron Alexis, had reported “hearing voices,” raising further questions about how he gained security clearance at the complex where he went on a shooting rampage.

Alexis, a Navy contractor and former Navy reservist, opened fire at the Naval Sea Systems Command on Monday, killing 12 people before police shot him dead.

Police in Newport, Rhode Island, say they were so concerned about Alexis’ behaviour on a business trip to the Navy Yard in August that they alerted the Navy police.

Alexis had told the police he believed people were following him and “sending vibrations into his body,” according to a Newport police report.

He said he had changed hotels twice to avoid the noise he heard coming through the floor and the ceiling of his rooms, and that the people following him were using “some sort of microwave machine” to prevent him from sleeping.

“Based on the naval base implications and the claim that the involved subject, one (Aaron Alexis) was ‘hearing voices,’ I made contact with the on-duty Naval Station police,” a Newport police officer wrote.

The Newport police report said Navy police had promised to check if Alexis was in fact a naval base contractor.

Asked for comment, a spokesman said the Navy was looking into the matter, without confirming any details.

In addition, CNN reported that Alexis had contacted two Veterans Administration hospitals recently and was believed to be seeking psychological help.

“Initial reports indicate that this is an individual who may have had some mental health problems,” U.S. President Barack Obama told the media.

“The fact that we do not have a firm enough background check system is something that makes us more vulnerable to these kinds of mass shootings,” he added.

The Navy gave Alexis an honourable discharge despite a series of eight to 10 misconduct charges, ranging from traffic offenses to disorderly conduct.

Using a valid pass as an information technology contractor with a private company, Alexis entered the Naval Sea Systems Command headquarters with a shotgun – bought legally in Virginia – and gained access to a handgun after he started firing, officials said.

He started picking off victims in a cafeteria from a fourth-floor atrium, witnesses said. Eight people were hurt, three with gunshot wounds, before Alexis was killed in a gun battle with police.

A Defense Department Inspector General’s report published on Tuesday revealed security lapses that allowed 52 convicted felons to gain access to Navy facilities because budget cuts had undermined vetting.

The Pentagon said it would review security at military installations around the world and the White House promised to review standards for federal government contractors.