LONDON — It took hundreds of British police officers and six months of meticulous investigation to charge two alleged Russian intelligence officers Wednesday in connection with the attempted murder of ex-spy Sergei Skripal and his daughter, Yulia.

But it is extremely unlikely that Alexander Petrov and Ruslan Boshirov — names which are believed to be aliases — will ever face justice.

Alexander Petrov and Ruslan Boshirov. Metropolitan Police / EPA

Officials said they have been able to piece together the exact movements of Petrov and Boshirov, the men who allegedly carried out a Kremlin-directed assassination attempt with Novichok, a military grade nerve agent.

Police revealed they know the airports, train stations, and hotels the suspects used before they attempted to kill Skripal, a former Russian intelligence officer convicted of spying for the West, and his daughter in their English hometown of Salisbury.

Around 250 detectives analyzed 11,000 hours of security footage and took 1,400 statements, according to Prime Minister Theresa May. She told lawmakers Wednesday that officers had been "working around the clock" and carrying out "painstaking and methodical work."

However, experts say the chance of arresting the men at the center of the investigation is virtually zero.

Alexander Petrov and Ruslan Boshirov at Salisbury train station on March 3, the day before the Skripals were poisoned. Metropolitan Poilce / EPA

Firstly, British prosecutors said they won't even ask for the men to be extradited because the Russian Constitution bars it. Article 61 is unequivocal: "A citizen of the Russian Federation may not be deported from Russia or extradited to another state."

U.K. officials have instead obtained a European Arrest Warrant, which means if either man steps inside the European Union they could be arrested.

"Should either of these individuals ever again travel outside Russia, we will take every possible step to detain them, to extradite them and to bring them to face justice here in the United Kingdom," May said.

But the likelihood of either suspect entering the E.U. — knowing it would likely lead to their immediate arrest — is low to say the least.

"If nothing else, it means these people, whatever their real names, will likely not be travelling much now," said Mark Galeotti, a senior nonresident fellow at the Institute of International Relations Prague. "Putting names and faces to the operatives helps the government make the case to its own people that this was a Kremlin hit."

Britain has been down this road before.

In 2006, two former Russian spies, Dmitry Kovtun and Andrei Lugovoi, allegedly carried out the poisoning of Alexander Litvinenko, another former intelligence officer, in London.

Alexander Litvinenko receiving treatment at a London hospital on Nov. 20, 2006. Natasja Weitsz / Getty Images file

An inquiry that took a decade to complete said the killing was "probably" directed by Russian President Vladimir Putin himself.

Not only were Kovtun and Lugovoi never brought to justice, but Lugovoi is now a lawmaker in the Russian Parliament, known as the Duma.

"Britain is realistic enough to know that there is no prospect of Russia extraditing its own hit men," NBC News security analyst Duncan Gardham said. "After all, they have been through this 10 years ago with the poisoners of Litvinenko."

May suggested that the order to assassinate the Skripals came from a similarly senior level of the Russian government.

"Only Russia had the technical means, operational experience and motive to carry out the attack," she said.

Sergei Skripal during a hearing in a Moscow court in 2006. Yury Senatorov / EPA file

The prime minister revealed that U.K. intelligence agencies have determined that the suspects were members of the GRU, the same Russian military intelligence service to which Sergei Skripal once belonged.

"This was not a rogue operation. It was almost certainly also approved outside the GRU at a senior level of the Russian state," she said.

Improbable as a prosecution may be, it is still essential for British investigators to carry out a thorough investigation, according to Gardham.

British officials "want the world to know they are prepared to wait, and that the British, like the Russians, have long memories," he said.

For British police "the motive is more basic," Gardham added. "The message is that they have identified suspects, that no one is untouchable, that they always get their man."

Neil Basu, assistant commissioner of the Metropolitan Police, on Wednesday talked through the suspects' movements on the weekend the Skripals were poisoned in early March.

A police officer stands outside the City Stay Hotel in London that was used by Alexander Petrov and Ruslan Boshirov. Toby Melville / Reuters

Basu called it “one of the most complex and intensive investigations we have undertaken in counterterrorism policing.”

He also spoke of evidence of what appeared to be a reconnaissance mission to the town of Salisbury, where the Novichok incident happened a day later.

The Skripals recovered, but two other people, Dawn Sturgess and Charlie Rowley, fell ill three months later apparently after coming into contact with some of the disposed substance.

Rowley was discharged from hospital but Sturgess later died.

Duncan Allan, a former official with the British Foreign Office who worked in British embassies in Moscow and Kiev, agreed there was little hope of catching the culprits.

But he said that despite the nonexistent chance of prosecution there was no question the crime should be investigated with no resource spared.

"Above all, there's a legal process that needs to be carried out here," said Allan, who's now an associate fellow at London's Chatham House think tank. "There's a criminal investigation that was underway and that's now reached its conclusion."

There is also a political case for charging the men, both in terms of putting more pressure on Russia, and convincing the British public that Moscow was actually behind the attack.

Britain expelled 23 Russian diplomats after the Skripal poisoning. More than 20 countries followed suit and ejected a total of 150 of the Kremlin's envoys.

Moscow reiterated its denial that it was behind the attack after the charges were announced Wednesday.

"The names published in the media, and the photographs, do not tell us anything," Russian Foreign Ministry spokeswoman Maria Zakharova told state media.

May told lawmakers that Russia has repeatedly "replied with obfuscation and lies."

Gardham said that the criminal charges might help to counteract "Russian disinformation suggesting that the plot is all cooked up by the British state."

He added, "With this level of detail, it is hoped it will dispel any lingering doubts."