As part of a Mafia investigation, undercover police monitored the star-studded "Ocean's Twelve" (search) production in Sicily after noticing that suspected mobsters were hanging around the set, an investigator said Tuesday.

The sequel to the heist hit starring George Clooney (search), Brad Pitt (search) and Julia Roberts (search) has been shooting in Italy for weeks, including this month in the seaside town of Scopello, western Sicily. An ongoing investigation of the local mob noticed that some Mafia suspects were stationed near the set, police official Giuseppe Linares said.

The mobsters may have been looking to extort the production or could have been worried that a secret alcove used to hide arms in the area could be discovered during the production, he said, by phone from Trapani, Sicily. Undercover and uniformed police secretly watched the set, Linares said.

The film's crew and cast were unaware of the investigation and of the local Mafia activity, Linares said, adding that authorities did not inform the filmmakers of the secret surveillance. No mob crimes were recorded at the set.

News of the unusual movie-monitoring emerged when police in Trapani announced Tuesday that they had arrested 19 alleged mobsters after a two-year investigation. The gang's alleged crimes revolved largely around the extortion of local businesses around the town of Castellammare del Golfo, near Trapani.

The 19 suspects and four others already in prison are accused of a range of crimes, from Mafia membership, to extortion, to the illegal possession of arms and explosives. Among the suspects arrested were two women accused of holding senior roles in the gang while their spouses were behind bars.