In this case, the radar image was of a participant in the public test of a three-year NATO project aimed at detecting explosives and weapons in subways, train stations and airports.

The project, dubbed "Dexter", follows decades of mass attacks on public transport, airports, stadiums and other venues.

NATO's main focus in the past three months has been European security following Russia's invasion of Ukraine. This is one of the alliance's scientific research schemes with a non-military focus.

NATO Assistant Secretary General for Emerging Security Challenges David van Weel said the technology, which is less invasive and more accurate than random searches, could have a "tangible impact".

"It could make attending a football game at stadiums safe and it could make airports more secure," he said.

In a subway corridor in the outskirts of Rome, where large screens showed colourful images from radar and laser systems, Deniz Beten, who leads NATO's Science for Peace and Security Programme, said Dexter was "just a prototype".

"We hope that in one or two years, we will be able to commercialise it for use in metros, airports or other infrastructures chosen by the countries," she said.

The project involves some 11 research institutions in NATO members France, Germany, Italy and the Netherlands, as well as others in partner members Finland, Serbia, South Korea and Ukraine.

- Sensor fusion -

Dexter integrates various sensor and software technologies to provide real-time information to police or security guards monitoring passengers in public spaces.

A radar system aimed at passing travellers produces high-resolution 2D and 3D images that highlight weapons in red, while another using laser can detect traces of explosives.

Software then processes the output from both sensors, and an alert is quickly sent to smart glasses worn by a police officer in a control room.

Henri Bouma, a senior scientist at the Netherlands' research organisation TNO, said the officer would be able to reach a suspect while they are "still near the sensors."

On a video screen an image of the suspect appeared with his face blurred -- a volunteer participating in the trial. Video images are not stored, Bouma said.

In the past month of tests, the two systems have had a 100 percent success rate detecting people with traces of explosives on their bodies or carrying weapons. Tests on bigger crowds gave a success rate of "over 99 percent," he said.

- Jump-start -

After decades of declining state funding for civilian R&D projects, programs like Dexter were important to fill gaps not served by the private sector and could jump-start their involvement, said Van Weel.

"Our aim is to stimulate the actual follow-up, which mostly will be by the private sector," he said, adding that Dexter's technology could be licensed.

Federico Angelini, a physicist with ENEA, Italy's national agency for new technologies, energy and sustainable economic development, said the Dexter researchers' job was to create the prototype.

"We demonstrate the principles but in the end it's not our job to know exactly which kinds of steps we need for a product to be on the shelf."