Rome ? Security experts from the U.S. State Department joined Italian investigators Tuesday to inspect a tunnel running alongside the U.S. Embassy in Rome in connection with an Italian probe of a group of Moroccans suspected of planning a chemical attack.

A hole was found in the tunnel after the Moroccan suspects were arrested last week.

“We take this investigation very seriously, we are cooperating very closely with Italian authorities,” U.S. Embassy spokesman Ian Kelly said Tuesday.

The experts inspected the tunnel for about 45 minutes. They made no comment as they went back inside the embassy.

The tunnel, running under Via Boncompagni, a street flanking the Embassy compound on fashionable Via Veneto, was marked on the suspects’ map of Rome’s underground utility lines. It contains electricity and telephone lines.

The U.S. Embassy said it is not yet known if the hole was made after recent, regular inspections by Italian utility crews of the tunnel, or if it was left over from previous maintenance work.

Police raids last week led to the arrest of eight Moroccans in the capital as well as the discovery of 8.8 pounds of a cyanide-based compound, potassium ferrocyanide, and firecrackers. Also found was a tourist map with the U.S. Embassy circled and municipal maps indicating the location of utility lines near the embassy, officials said.

Italian news reports have said that investigators believe a chemical attack on the Embassy’s water system was being plotted.

But the U.S. Embassy officials said Monday there was “no hard evidence” an attack being planned on the Embassy’s water supply.

There’s a lot of circumstantial evidence, a U.S. official said, adding that they don’t know the aim of the Moroccans.

The Moroccans have denied any wrongdoing, saying they don’t know how the cyanide compound got into the apartment.

They are charged with subversive association.

A ninth suspect, also Moroccan, is being detained in the southern town of Reggio Calabria.