Scotland Yard have arrested a woman under terror laws today as part of a wider investigation into an alleged "super-secret" Facebook plot to blow up an Italian synagogue.
The 40-year-old suspect was held in a pre-dawn swoop on her south London home in relation to extremist material posted online.
Italian police said she may have been in contact with a 20-year-old Moroccan accused of making a "detailed plan" for a terror attack on the Via della Guastalla in Milan.
Officers in London said they were liaising with Italian counterparts to establish any links. She is not thought to be directly involved in the alleged attack plot.
The man arrested in Brescia, northern Italy, was using his "exceptional" computer skills to create "super-secret" groups on Facebook to plot an atrocity, Italian police claim.
Using his "exceptional" computer skills, he created "super-secret" groups on Facebook that could be accessed only through a complicated system of controls that he had put in place, state police said.
The woman was held under Section 58 of the Terrorism Act on suspicion of "possessing a document or record containing information of a kind likely to be useful to a person committing or preparing an act of terrorism."
Searches are ongoing at the woman's south London address as she continues to be questioned.
"Officers from the MPS (Metropolitan Police Service) Counter Terrorism Command have arrested a 40-year-old woman on suspicion of offences contrary to Section 58 of the Terrorism Act 2000 today," a Scotland Yard statement said.
"The woman was arrested in south London at about 4.30am and taken to a local police station where she remains in custody.
"A search warrant was executed at a residential premises in connection with the arrest, and a search continues."
Officers in Italy said the man had been living in the province since the age of six and "had the job of training people in the use of weapons and explosives for terror operations".
"In this online arena, members could share instructions on how to assemble explosive devices, what chemical ingredients could be bought and the use of weapons," a state police statement translated into English said.
"Anti-terrorism officers intercepted a message from the Moroccan in which he spoke of a 'Jihad mission'."
Police allegedly found a document saved on his computer, "noting every detail in view of the planned attack on the Milan synagogue: security measures that were in place, police on duty, possible obstacles and possible access routes".
Police said they were aware the Moroccan could have accomplices, including abroad.
"Parallel investigations are under way in England and the United States, where there are people who were in contact with the man over the internet," the statement said.
"The investigation shows how cyberspace can be the optimal environment for extremists and terrorists for planning jihad because of the ease with which they can establish virtual links and feel safe while operating due to the certain amount of anonymity it gives them."