Police have identified 16 people, 15 of whom are in custody, as members of a violent underworld gang that carried out contract killings and is suspected of being behind the kidnapping earlier this year of ferry firm owner Pericles Panagopoulos, who was released after his family paid a ransom of 30 million euros.

Officers revealed on Saturday that five of the suspects had already been serving jail sentences, that 10 had been arrested in the last few days and that one person remained at large.

Attica Police Chief Yiannis Dikopoulos said that the gang’s mastermind was Panayiotis Vlastos, who is serving sentences for murder, racketeering and other crimes in Trikala Prison. Vlastos was allegedly aided by four other inmates: Vassilis Stefanakos, another notorious underworld figure, Yiannis Skaftouros, Constantinos Andreou and Serbian Dejan Ivanov.

Police are still searching for one suspect, Panayiotis Soidelis. Of the remaining 10 suspects, two are the wives of alleged gang members.

Vlastos is said to have organized the Panagopoulos kidnapping, which took place in January, from prison. Another eight people, including his wife, are alleged to have taken part in the abduction, during which the owner of Super Fast Ferries was held for eight days before the ransom was paid and he was freed unharmed.

Dikopoulos revealed that the suspects were arrested after the National Intelligence Service helped police gather enough incriminating evidence. Officers are now examining this information in the hope that it will confirm suspicions that the gang carried out several contract killings.