The CIA believes it is unlikely that Russia or another foreign adversary is using microwaves or other forms of directed energy to attack hundreds of American officials who attribute symptoms associated with brain injuries to what's come to be known as "Havana syndrome."

The agency's findings, according to one official familiar with the matter who spoke on condition of anonymity to discuss the intelligence, drew immediate criticism from those who have reported cases and from advocates who accuse the government of long dismissing the array of ailments.

Investigators have studied hundreds of reported cases reported globally by U.S. intelligence officers, diplomats, and military personnel and whether the injuries are caused by exposure to forms of directed energy. People affected have reported headaches, dizziness, nausea, and other symptoms consistent with traumatic brain injuries. 

Most cases under review by intelligence officers have been linked to other known medical conditions or to environmental factors, the official said, adding that in some cases, medical exams have revealed undiagnosed brain tumors or bacterial infections. 

A few dozen cases are unresolved and remain under active investigation, the official said. The involvement of a foreign adversary has not been ruled out in those cases. NBC first reported the CIA's interim findings. 

In a statement, CIA Director William Burns said the agency's commitment to its officers' health was "unwavering."

"While we have reached some significant interim findings, we are not done," Burns said. "We will continue the mission to investigate these incidents and provide access to world-class care for those who need it."

Mark Zaid, a Washington lawyer representing more than 15 officers who have reported cases, asserted that the CIA is having a "revolt within its workforce" among people who do not want to take overseas assignments for fear of being attacked. 

"No reasonable person is asserting an exact conclusion exists that points to a specific culprit or weapon, but the issuance of this interim report was unnecessary and premature," Zaid said in a statement. 

"Havana syndrome" cases date to a series of reported brain injuries in 2016 at the U.S. Embassy in Cuba. Incidents have been reported by diplomats, intelligence officers, and military personnel in the Washington area and at global postings. Russia has long been suspected by some intelligence officers of using directed energy devices to attack U.S. personnel. 

Democrats and Republicans have pressed President Joe Biden's administration to determine who and what might be responsible and to improve treatment for victims. Biden last year signed a bill intended to provide better medical care for victims. The State Departmen t also appointed a new coordinator for its review into cases after victims criticized the previous coordinator. 

At a news conference Thursday in Berlin, Secretary of State Antony Blinken said that after meeting with people who had reported cases, "there is no doubt in my mind that they have had real experiences, real symptoms and real suffering." 

"We are going to continue to do everything we can, with all the resources we can bring to bear, to understand, again, what happened, why, and who might be responsible," Blinken said. "And we are leaving no stone unturned."