This combination of photos made in Paris on Monday shows the suspected mastermind of the November 13 Paris attacks, 28-year-old Belgian IS group leading militant Abdelhamid Abaaoud (top right), French Bilal Hadfi (bottom centre), one of the suicide bombers who blew himself outside the Stade de France stadium, Samy Amimour (top left), one of the suicide bombers who attacked a Paris concert hall, suspect at large French Salah Abdeslam (bottom left), an unidentified man (top centre) suspected of being involved in the attacks, and the unidentified third suicide bomber (bottom right) of the attack at the Stade de France stadium.

Reuters/Bari, Italy

Suspected Paris attacker Salah Abdeslam travelled through Italy in August with a companion, but his presence caused no alarm because he was not a wanted man at the time, an investigative source said on Monday.

Separately, Bari prosecutors have opened an investigation into suspected Islamist militants who passed through the port on some five different occasions between February and August on their way to Greece, another investigative source said.

This investigation has no apparent connection to the Paris attacks, the source said. Prosecutors suspect that militants, using false documents, are travelling through Italy en route to Syria.

The cases show the difficulties of policing and collecting intelligence in the 28-member European Union, where each country's security forces work independently and have different priorities.

Abdeslam is a French citizen and could therefore travel freely between countries in the European Union's Schengen area, where there are no border controls.

Since the attacks, some countries have imposed temporary controls and there have been calls for the borderless system to be scrapped.

Abdeslam, whose brother blew himself up in the Paris attacks, has been on the run since the assault that killed 130 and is the focus of a massive manhunt.

He boarded a ferry in the south-eastern Italian port of Bari on August 1 en route to Patras, Greece, the source said, with another man. Abdeslam returned from Patras to Bari by ferry on August 5. The source gave no further details.

In an interview with an Italian website on Monday, Interior Minister Angelino Alfano confirmed Abdeslam had passed through Italy.

"The point is that they were citizens with European passports and ... they were not wanted" by police, Alfano said.