A US intelligence report by a panel of expert scientists has named pulsed electromagnetic energy and ultrasound as plausible causes for the mystery Havana Syndrome symptoms suffered by US diplomats and spies in recent years.

The report found that a group of cases could not be explained by health or environmental factors or by psychosomatic illness. It also said that devices exist with “modest energy requirements” which were concealable and could produce the observed symptoms and be effective over hundreds of meters or through walls.

The panel, established last year by the director of national security, Avril Haines, and the CIA director, William Burns, said the investigation was not tasked to identify a culprit, but in a statement accompanying the report, Haines and Burns said it would help sharpen the search for the origins of the mysterious ailments.

“We will stay at it, with continued rigor, for however long it takes,” they said.

In a redacted executive summary of its report published on Wednesday, the panel of experts said that the signs and symptoms of the syndrome were “diverse and may be caused by multiple mechanisms” but that a subset of them “cannot be easily explained by known environmental or medical conditions”.

The expert panel report clashed in tone to a briefing given by the CIA last month which said that in the majority of cases, there was no sign of a malignant campaign by a foreign power. That briefing, possibly aimed at limiting the further spread of psychosocial illness among US officials, did caution that in some two dozen cases the symptoms could not be easily explained.

The latest panel narrowed its review of the hundreds of reported cases by defining four “core characteristics”: the sudden onset of sound or pressure in one ear or one side of the head; near simultaneous symptoms like vertigo, loss of balance and ear pain; a “strong sense of locality or directionality” – the feeling of being assailed from a particular direction; and the absence of known environmental or medical conditions that might otherwise explain the symptoms.

The summary did not go into details of cases, but the description of symptoms matched the earliest cases in 2016 reported among US and Canadian diplomats stationed in Havana, from which the syndrome got its name.

The expert panel found “the combination of the four core characteristics is distinctly unusual and unreported elsewhere in the medical literature, and so far have not been associated with a specific neurological abnormality.”

When looking at probable causes, the panel – which includes experts on medicine and engineering – pointed towards some form of external energy source.

The report found that: “Pulsed electromagnetic energy, particularly in the radio frequency range, plausibly explains the core characteristics, although information gaps exist.”

It added that “sources exist that could generate the required stimulus, are concealable, and have moderate power requirements.”

Using what the report described as “nonstandard antennas and techniques”, electromagnetic pulsed energy could be directed at a target “through air for tens to hundreds of meters, and with some loss, through most building materials”.

Pulsed electromagnetic energy had been “credibly demonstrated” to cause the observed symptoms, the panel found, and that “persons accidentally exposed to radio frequency signals described sensations similar to the core characteristics”.

However, the report said there was a “dearth of research” on the effects of such pulsed electromagnetic energy on humans.

Engineers who had been working on a potential weapon for the US marines two decades ago, known as Medusa, said that one of the reasons it was discontinued was that it was ethically impossible to conduct human tests on the prototype.

A large part of the section on pulsed electromagnetic energy was redacted in the published expert panel report.

The executive summary said that ultrasound energy was also a plausible explanation for the symptoms but only in situations where the source was close to the target.

“The required energy can be generated by ultrasonic arrays that are portable, and produce a tight beam,” it found, but noted that “ultrasound propagates poorly through air and building materials”.

The report said that psychosocial factors could not explain the cases exhibiting the four core characteristics of Havana Syndrome but could have made them worse. Such factors could also explain other incidents which “could be due to hypervigilance and normal human reactions to stress and ambiguity, particularly among a workforce attuned to its surroundings and trained to think about security”.

A victims group issued a statement on Wednesday arguing the new report “reinforces the need for the intelligence community and the broader US government to redouble their efforts to fully understand the causes of … Havana Syndrome”.

Mark Zaid, a Washington attorney representing victims from multiple federal agencies, said: “These piecemeal agency reviews at times reveal inconsistent and even contradictory results that undermine the effort to resolve this controversy.”