WASHINGTON — A month before a murderous rampage at the Washington Navy Yard, Aaron Alexis called the police in Rhode Island to complain that he had changed hotels three times because he was being pursued by people keeping him awake by sending vibrations through the walls.

When officers came to his hotel room early on Aug. 7, Mr. Alexis told them that a person he had argued with at an airport in Virginia “has sent three people to follow him” and that they were harassing him with a microwave machine, according to a Newport, R.I., police report. Mr. Alexis said he had heard “voices speaking to him through the wall, flooring and ceiling,” the report said.

Mr. Alexis told the police he was a Navy contractor, and then twice that month he sought treatment from the Veterans Affairs Department for psychiatric issues, according to a senior law enforcement official. But it did not raise a red flag that might have prevented him from entering the military base in Washington where, the authorities say, he killed 12 people on Monday.

On Wednesday, Mr. Alexis’s mother apologized to the victims in a brief statement she read from her Brooklyn home.

“I don’t know why he did what he did, and I will never be able to ask him why,” Cathleen Alexis said. “Aaron is now in a place where he can no longer do harm to anyone, and for that I am glad. To the families of the victims: I am so so very sorry that this happened. My heart is broken.”

The episode in Rhode Island adds to a growing list of questions about how Mr. Alexis, who had a history of infractions as a Navy reservist, mental health problems and run-ins with the police over gun violence, gained and kept a security clearance from the Defense Department that gave him access to military bases, including the navy yard, where he was shot to death by the police.