British and American authorities have been given several chemical analyses of a substance believed to be a novichok nerve agent produced in Russia’s closed Shikhany military facility, a Russian lawyer has told the Guardian.

Boris Kuznetsov, who fled Russia in 2007, said he had handed British diplomats the police case files from the 1995 murder of a Russian banker and his secretary with a toxic substance, which scientists have identified as a product of the Soviet-designed Foliant programme.

A related nerve agent was used in the Salisbury poisoning last month.

The documents, some of which had previously been leaked by the Russian newspaper Novaya Gazeta, were given to British diplomats in Latvia in March, Kuznetsov said. He told the Guardian he had also handed the case files to US authorities.

Among the documents, which have been seen by the Guardian, are the results of a mass spectrometry and an infrared spectroscopy of the poisonous substance, which was scraped off a telephone receiver used by the businessman Ivan Kivelidi and his secretary. Both died in agony.

The files may provide rare physical evidence about chemical weapons production at Shikhany. The British case has so far relied more heavily in public on circumstantial evidence and secret intelligence.

Alastair Hay, an environmental toxicologist at the University of Leeds who investigated the use of chemical weapons against Iraqi Kurds in Halabja in 1988, said “the data would be very helpful” after being given a description of the files.

He said an analysis of the lab data would be necessary to make a further assessment, but the hydrolitic process used in several of the analyses of the suspected novichok, and the quality of the mass spectrometry data, would be of particular importance.

On Thursday, the Times reported that British intelligence had identified Shikhany as the source of the nerve agent used in the Salisbury attack, during a briefing for the country’s allies.

Kuznetsov said: “If the substance synthesised by Rink and the substance which poisoned Skripal are identical, then that means that the attempted murder of Skirpal was made with the Russian government’s knowledge.”

But with a number of labs producing at least five variants of novichok across Russia, and a lapse of two decades between the cases, the ultimate use of the data from the 1995 murder may be limited in relation to Skripal.

British scientists at Porton Down said this week that they had not determined the country of origin of the nerve agent, explaining it was “not our job to say where it was manufactured”. The Foreign Office subsequently deleted a tweet that said the nerve agent was “produced in Russia”, saying the remarks had been truncated.

A representative for the Russian foreign ministry said that the UK had its own “test tube of shame”, a reference to the evidence shown by the then US secretary of state Colin Powell to the UN security council in support of the Iraq war.

The Kivelidi case files also included blood work analyses, largely testing for the presence of heavy metals. Investigators initially believed he had been poisoned with cadmium.

During a police interrogation, Vladimir Uglev, a scientist who worked on the programme, had identified the poisonous substance in the killing as one of the nerve agents produced at his lab in Shikhany, he told the Guardian.

Leonid Rink, a former employee of the chemical weapons facility, admits in case files seen by the Guardian and first reported by Reuters to enlisting a scientist to develop a batch of the nerve agent in his garage and sell it to an organised crime group. He was caught before the murder of Kivelidi, but the nerve agent was later used in the killing.

Russia did not publicly declare its Foliant programme before signing the 1993 chemical weapons convention, limiting known data about the nerve agent’s properties. Some chemical formulas were published in a 2008 book by the whistleblower Vil Mirzayanov. The US may have collected physical evidence of the nerve agent during its cleanup of the Nukus chemical weapons testing facility in Uzbekistan in 1999.

Otherwise, little is publicly known about the programme. Russia’s ambassador to the UK has denied Moscow ever developed a nerve agent called novichok. The argument appeared to be semantic.

At the same time, Russia has said that because the chemical formula for the nerve agent was well known, any country could have synthesised the poison used in Salisbury.

But data from a nerve agent produced in Shikhany may show some differences between it and comparison samples possibly produced in labs outside Russia.

Uglev had previously told the Guardian it would be difficult for investigators to pinpoint the source of a nerve agent without any previous data on it.

“They have the footprint of the substance in Salisbury … but no data about the substance [its fingerprint] in the database, so how can they say where it is from?” he wrote.