A fifth suspect has been arrested in connection with arson attacks on properties linked to Sir Keir Starmer.
The Metropolitan Police said a 19-year-old man has been released under investigation after being taken to a police station in Essex for questioning on Wednesday.
The suspect, of Harlow, Essex, was arrested on suspicion of conspiracy to commit arson, the force said.
Three men are due to stand trial at the Old Bailey in April accused of targeting two properties and a car following the arson attacks in May last year.
A fourth man has been released without charge, a force spokesman said.
A court previously heard two of the blazes occurred in Kentish Town, north London.
One of the fires took place in the early hours of May 12 at the home where Sir Keir lived before he became Prime Minister and moved into Downing Street.
A car was set alight in the same street four days earlier on May 8.
The third incident took place on May 11 at the front door of a house converted into flats in Islington.
Ukrainian nationals Roman Lavrynovych, 21, and Petro Pochynok, 35, both previously pleaded not guilty to conspiracy to commit arson with intent to endanger life between April 1 and May 13 during a hearing at the Old Bailey in October last year.
Romanian national Stanislav Carpiuc, 27, pleaded not guilty to the same charge on November 28, a court spokesman said.
Prosecutors previously said the case was not being treated as terrorism-related.
Lavrynovych, of Lewisham, south-east London, Carpiuc, from Romford, east London, and Pochynok, of Islington, north London, have been remanded into custody until their trial on April 27.
The Met said the investigation, which is being led by counter-terrorism officers, remains ongoing.
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