Italian security forces were rounding up 18 Islamic extremists Friday who prosecutors said were behind a failed 2010 plot to attack the Vatican as well as a bombing at a Pakistan market that killed more than 100 a year earlier.

Prosecutor Mauro Mura told reporters in Cagliari, Sardinia, on Friday that wiretaps indicated the suspected terrorists, including two former bodyguards for Usama bin Laden, planned a bomb attack at the Vatican and went as far as to send a suicide bomber to Rome. Mura said the attack plans never went further and that the suicide bomber left Italy, though it wasn't clear why.

“From what it appears, this concerns a hypothesis that dates from 2010 which didn’t occur.”

- Federico Lombardi, Vatican spokesman

“We don’t have proof, we have strong suspicion,” Mario Carta, head of the police unit leading the investigation, said when asked for more details on a possible attack against the seat of the Catholic church.

Authorities said nine suspects had been caught, and another nine were being sought, three of whom were believed to still be in the country.  One of the suspects arrested Friday had a construction business in Sardinia that participated in work for a Group of Eight summit planned for Sardinia but that was later moved to quake-stricken Aquilia, in Abruzzo to boost reconstruction. Another was an imam in the northern province of Bergamo.

Vatican secretary of state Pietro Parolin said the threat is chilling, even if it is old.

"We are all exposed and we are all afraid," Parolin said. "But the pope is very calm for this, it's enough to watch him meeting people with great clarity and serenity."

At the time of the suspected plot to bomb the Vatican, Pope Benedict XVI was still reeling from the effects in the Muslim world of a 2006 speech in Regensburg, Germany, in which he quoted a Byzantine emperor who characterized some of the teachings of the Prophet Muhammad as "evil and inhuman," particularly "his command to spread by the sword the faith."

While relations with the Muslim world were eventually repaired, tensions flared again in 2011 when Cairo's al-Azhar institute, the pre-eminent theological school of Sunni Islam, suspended interfaith talks with the Vatican after Benedict called for greater protections for Egypt's minority Christians.

More recently, Italian officials have made clear they take seriously the threat of the Islamic State group to conquer Rome and the seat of Christianity. Security has been beefed up at the Vatican and the head of the Swiss Guards has said they are ready but that they have no information about an imminent threat. Still, Pope Francis himself has said he realizes he may be a target but that he fears mostly for the innocent crowds who come to see him every time he's in public.

The investigation was launched in 2005, but Mura said it was slowed when news of the investigation leaked to the media, alerting the suspects that they were being watched.
Arrests warrants for the suspects, who were also linked to the deadly 2009 market bombing in Peshawar that alleged Islamic extremists, were being executed around the country.

Anti-terror police on the island of Sardinia said they were executing arrest warrants for the suspects, although some were believed to have fled the country. Police said some were responsible for “numerous bloody acts of terrorism in Pakistan,” including a deadly October 2009 explosion in a Peshawar market that killed more than 100 people. Telephone wiretaps indicated that two of the suspects were part of a network of people who protected Bin Laden in Pakistan, a police statement said.

Police said the aim of the terror network was to create an insurrection against the Pakistani government.

Pasquale Errico, the police chief in the city of Sassari, said the warrants were being executed throughout Italy. The suspects were also being sought for financing terrorist movements in Afghanistan and Pakistan.

Sardinian daily L'Unione Sarda reported that one of the suspects ran a construction business in Olbia.

"There is the hypothesis that he was recruiting immigrants who arrived here by airplane with false documents and involved them in illegal activities," the paper's editor Anthony Muroni told Sky TG24.

The Associated Press contributed to this report