Italian police said Wednesday they have arrested four Moroccans who were in possession of large quantities of cyanide and maps of the Italian capital and its water supply network. The men were believed to have been plotting a terror attack in Rome.

Italian police said the four Moroccans were found with four kilos of powdered cyanide. Maps in their posession marked the location of the U.S. embassy and other sites.

Police found false immigration papers in their apartments.

The men, aged between 30 and 40, were arrested in an early morning raid Tuesday on the outskirts of Rome. Italian police say they had been following the Moroccans for days. Their arrest was linked to that of three other Moroccan nationals last week.

The operation has been described as the biggest investigation into suspected Islamic terrorists in Rome. But it is unclear whether the Moroccans may have been planning to contaminate the capital's water system or target the U.S. embassy.

Last year, the embassy was closed to the public for three days because of a terrorist threat. Following the September 11 attacks in New York and Washington, the State Department asked Italian authorities to step up security around American symbols in Italy.

Investigators believe the Moroccans are linked to what is thought to have been an Islamic terrorist cell based in Milan. Police dismantled the cell last year, arresting seven Tunisians suspected of having links with Osama bin Laden's al Qaida organization.

The Tunisians are now on trial in Milan. They are charged with a number of crimes, including trafficking in weapons and explosives and helping illegal immigrants enter the country.

The United States warned last year that Milan's Islamic Cultural Center was al Qaida's main logistic base in Europe. The allegation has been strongly denied by the center's Muslim leader.