“Had this information been reported, properly adjudicated and acted upon,” the review said, Mr. Alexis’ “authorization to access secure facilities and information would have been revoked.”

In 2004, three years before enlisting as a Navy reservist, Mr. Alexis brandished a .45-caliber pistol and fired at the tires of a construction worker’s car. In September 2010, he was arrested by the police in Fort Worth after he fired through the ceiling of his residence. But commanders stopped efforts to force Mr. Alexis out of the Navy with a less-than-honorable discharge because the police declined to pursue the case, and it was not entered into his military file.

In August 2013, a few weeks before the shooting, Mr. Alexis told the police in Rhode Island that he was hearing voices sent by a “microwave machine,” prompting the authorities to fax a report to a naval police in Newport, where Mr. Alexis was working temporarily as a contractor. But the naval police did not alert superiors in Washington.

At the time, Mr. Alexis’ company temporarily withdrew his access to classified information and contacted his mother, who told them, according to a subsequent company investigation, that her son was paranoid and that the microwave episode was not the first of that kind he had experienced. But the report said the company restored his secure access after concluding that the information was based on rumor.

Paul N. Stockton, a former assistant secretary of defense who worked on the independent review, told reporters at the news conference that for decades the military has operated on the premise that “if we build a fence around us, we’ll be secure.” But he said that approach “is outmoded, it’s broken, and it needs to be replaced.” Much more needs to be done, he added, to make sure that people working on American military bases are not dangerous to others.

The independent review, led by Mr. Stockton, also suggested conducting more background checks on people who already have security clearances.

Currently security clearances are reviewed every five or 10 years, depending on the level of clearance. “The assessment is that that approach limits our ability to understand the evolution that may occur in a person’s life that may have them evolve from a trusted insider to an insider threat,” said Marcel Lettre, the principal deputy under secretary of defense for intelligence, who spoke at the news conference.