Rome ? Authorities were detaining four Moroccan immigrants Wednesday who may have been planning a terror attack on the U.S. Embassy in Rome, police said.

In a raid on the men’s apartment in a Rome suburb, the police found maps highlighting the location of the American embassy and diagrams of the city’s water supply system as well as 10 pounds of a chemical substance containing cyanide. The chemical, potassium ferrocyanide, would not have posed a threat to the city’s water system.

The embassy, located in a former royal villa on Rome’s famed Via Veneto, came under threat of an attempted al-Qaida terror attack a year ago.

The embassy was forced to close for a few days in January 2001 after U.S. intelligence picked up warning signs of a possible attack. During the next 10 months, Italian police in Milan arrested seven Tunisian immigrants with alleged links to al-Qaida who are believed to have been involved in the plot.

The seven are now being tried in Milan on charges of trafficking in false documents, violating immigration laws and criminal association with the intent to obtain and transport weapons, explosives and chemicals.

“The embassy of the United States of America compliments the Italian police and security forces for their excellent work concerning the most recent security threat,” said a statement issued by the embassy, which maintained a normal work schedule Wednesday.

“The case in question is still an ongoing investigation, and therefore it would be premature to comment any further at this time,” the statement added.

Karyn Mullen-Posner, an embassy spokeswoman, said she had no information about any connection between Tuesday’s arrests and the earlier plot.

But police sources confirmed that at least one of the Moroccans appears to have ties to the Milan group, whose activities were centered around a Milan mosque called the Islamic Cultural Institute.

The key figure in the Milan trial is Essid Sami Ben Khemais, 33, who police say provided important logistical support to the shadowy network of al-Qaida cells that sprouted across Europe in the late 1990s.

Ben Khemais first came to the attention of U.S. officials about two years ago when intelligence sources warned that a man using the alias Omar al Muhajar was “joining a group of three Islamic extremists who were linked to the Osama bin Laden organization and, from Afghanistan, were planing vague actions against American targets in Italy,” according to court documents. Muhajar turned out to be Ben Khemais.

Wiretap and listening-device evidence presented in court earlier this month indicated that Ben Khemais and his associates had a working familiarity with bomb-making techniques and crude chemical weapons.