LONDON — In March, when British detectives began their investigation into the poisoning of Sergei V. Skripal, the former Russian spy, they had little to work with but mounds of CCTV footage. Heads bent over their desktop computers, they began the unglamorous work of poring through it, looking for an assassin.
Britain is one of the most heavily surveilled nations on earth, with an estimated one surveillance camera per 11 citizens. It has cutting-edge technology for visually identifying criminals, and software so sensitive it can scan an airport for a tattoo or a pinkie ring. And then there is that team of genetically gifted humans known as “super-recognizers.”
On Wednesday, the authorities announced that the effort had paid off: Two Russian intelligence officers had been charged with attempted murder, the first criminal charges in a case that has driven a deep wedge between Russia and the West.
Investigators released a cache of evidence, including security camera images that captured the progress of the two men from an Aeroflot flight to the scene of the crime, and from there back to Moscow. They also released photographs of the delicate perfume bottle that was used to carry a weapons-grade nerve agent, known as Novichok, to the quiet English city of Salisbury where the attack took place.
In the days leading up to the March 4 poisoning, the same two Russian men kept popping up on cameras.
“It’s almost impossible in this country to hide, almost impossible,” said John Bayliss, who retired from the Government Communications Headquarters, Britain’s electronic intelligence agency, in 2010. “And with the new software they have, you can tell the person by the way they walk, or a ring they wear, or a watch they wear. It becomes even harder.”
The investigation into the Skripal poisoning, known as Operation Wedana, will stand as a high-profile test of an investigative technique Britain has pioneered: accumulating mounds of visual data and sifting through it.
Neil Basu, Britain’s top counterterrorism police official, broke months of silence in a hastily convened Scotland Yard news conference on Wednesday, taking the unusual step of stripping journalists of their electronic devices to keep the news under wraps until arrest warrants for the two men, Alexander Petrov and Ruslan Boshirov, had been issued. Two hours later, Prime Minister Theresa May announced that British intelligence services had identified the men as officers in the G.R.U., Russia’s military intelligence service.
Russian officials responded witheringly, declaring in a Foreign Ministry statement that “we decisively reject these insinuations.”
“It is impossible to ignore the fact that both British and American colleagues act according to the same scheme: Without bothering themselves to produce any evidence, they announce a list of some ‘Russian agents’ in order to justify London and Washington’s witch hunt,” said Maria Zakharova, a Foreign Ministry spokeswoman.
Mr. Bayliss said that all along, investigators have been acutely aware that the suspects would be protected in Russia and never tried, though Interpol red notices and domestic and European arrest warrants were issued
“There are a lot of people who would sort of give up on it, because what’s the point?” he said. “They’re in Russia, we’re not going to get them back. But the thing is, once you’ve got it to that point, that means those people can’t leave Russia.”
Beyond that, Mr. Bayliss said, “there is a satisfaction of getting to the truth, to be able to prove to the Western world that the Russians did this.”
The day of the attack, Mr. Skripal and his daughter, Yulia, were found barely conscious on a bench beside the Avon River. (They both recovered, but months later, two Britons, Dawn Sturgess and Charlie Rowley, fell ill after being exposed to the poison. Ms. Sturgess died.)
In the days that followed the Skripal attack, investigators began by collecting 11,000 hours of video from ports, train stations, shop windows, car dashboards and the roadways around Mr. Skripal’s house.
Before searching for a needle, investigators said wryly, they first had to build their own haystack.
The investigation drew on some of Scotland Yard’s most storied assets, like its Super-Recognizer Unit. Its officers are selected for their superior ability to remember faces — the opposite of prosopagnosia, also known as “face-blindness.”
“They don’t concentrate on the obvious: the graying hair or the mustache or the glasses,” the unit’s founder, Mick Neville, told Sky News last week. “They look at the eyes, the mouth, the ears — the things that don’t change. They can recognize a face from the tiniest glimpse of part of it.”
In cases such as the Skripal investigation, which begin with an enormous pool of potential suspects, super-recognizers can help by singling out people who seem to move suspiciously, experts say. Local police officers are often brought in to help them eliminate bystanders, like small-time drug dealers, who may also appear suspicious.
Those results were then overlaid with passport data for Russians who left the country shortly after the poisoning, bringing the pool of suspects down to a manageable number. The police were able to cross-reference suspects in other ways, mapping mobile phone and bank card use, for example.
“It’s a bit like a funnel, the top of the funnel has a vast amount going in, and by the time the liquid comes out at the bottom, it narrows down to a tiny stream,” Mr. Bayliss said.
Investigators had one bit of luck: Heavy snow fell through the weekend of the attack, reducing the number of people on the streets.
A big breakthrough took place nearly two months after the Skripals were poisoned, when the police arrived at the City Stay Hotel in East London, where the two suspects had spent the two nights before the attack. Officers took samples from the room where the two men had stayed, and sent them for laboratory testing. Two of them showed trace contamination for the nerve agent used in the attack.
On Wednesday, as news of the charges spread, neighbors peered curiously at the building, which had smeared windows and dingy artificial grass.
“I just got a shiver, a cold shiver,” said Debbie Weekes, 47, who lives nearby. “It’s just shocking, I’m at a loss for words. You never know who’s around.”
Some wondered why they had not received a warning in May, when the police found the nerve agent traces in the hotel.
“Obviously we don’t feel safe,” said Shehan Ravindranath, 43, the manager of a supermarket across the street. “We can only take protection if we know about it.”
In Salisbury, though, the announcement about the charges was greeted with relief. Matthew Dean, the head of Salisbury’s City Council and owner of a local pub, the Duke of York, said he hoped it would put to rest conspiracy theories circulating about the crime.
“This is a piece of closure,” he said.
Ceri Hurford-Jones, the managing director of Salisbury’s local radio station, saluted investigators for their “sheer skill in getting a grip on this, and finding out who these people were.”
It may not have been the stuff of action films, but Mr. Hurford-Jones did see something impressive about the whole thing.
“It’s methodical, plodding,” he said. “But, you know, that’s the only way you can do these things. There is a bit of Englishness in it.”