Counter-terror police officers are understood to be probing possible Russian involvement in a series of "arson attacks" at properties and a car linked to the Prime Minister.
The trio of attacks on Sir Keir Starmer's family home, a flat he once owned, and a car he once owned, were carried out over a four-day period in early May.
So far, three men - two Ukrainians and a Ukrainian-born Romanian - have been charged with "conspiracy to commit arson with intent to endanger life".
In order of when they were charged, the trio are: Roman Lavrynovych, Stanislav Carpiuc and Petro Pochynok.
RIGHT TO LEFT: Roman Lavrynovych, Stanislav Carpiuc and Petro Pochynok
They have been charged with criminal, not national security offences.
Now, senior Whitehall figures have told the Financial Times that authorities are looking into whether "actors in Russia" may have recruited the trio.
These may not necessarily be Kremlin-linked "actors", and the suspects may have been unaware of their alleged nationality.
Discussions are underway on Britain's best course of action if the Russian link is true, the sources said.
The Russian 'actors' may not be linked to the Kremlin, sources said
GETTY
But a Government official has cautioned that several different versions of events remain under investigation, and nothing has been ruled out.
In court, prosecutor Sarah Przybylska has said the alleged conspiracy is "unexplained".
Meanwhile, counter-terrorism police are keeping an open mind about a potential motive.
None of the suspects have so far shown any ideological alignment with Russia.
The three all remain in custody - and are due to next appear at the Old Bailey on June 6.
The trio of attacks on Sir Keir Starmer's family home, a flat he once owned, and a car he once owned, were carried out over a four-day period in early May
PA
The first blaze involved a car once owned by Starmer on May 8.
The vehicle, which he had previously sold to a neighbour on the street, was set alight overnight.
The second erupted at a flat in which the future Prime Minister lived during the 1990s in north London on May 11.
The third took place at his family home in Tufnell Park, also in north London, on May 12.