Italian prosecutors have revealed they think Islamic extremists were ready to plant a bomb in the Holy See in 2010 but are unclear why the attack was not carried out.
Prosecutor Mauro Mura told a press conference a suicide bomber had arrived in Rome to blow himself up inside the Vatican.
He said although secret recordings gave "signals of some preparation for a possible attack" it was not known why the bomber left Rome without committing the crime.
The announcement was made as Italian police arrested 18 suspected extremists including two alleged bodyguards for Osama bin Laden.
But the Vatican has downplayed the revelations and said security will not be increased.
Spokesman, Fr Federico Lombardi, said: "From what it appears, this concerns a hypothesis that dates from 2010 which didn't occur.
"It has therefore no relevance today and no reason for particular concern."
The Vatican has been recently threatened by Islamic extremists.
Last year a Islamic State newspaper featured an image of the Vatican with the IS flag flying.
Security has previously been increased at the Holy See and the head of the Swiss Guards has said soldiers are prepared.