ROME (Reuters) - Italian police have searched three houses in and around Rome, where the man suspected of killing 12 people last week at a Christmas market in Berlin may have spent time, a judicial source said on Thursday.

Anis Amri, a Tunisian, first arrived in Europe by boat to the Italian island of Lampedusa in 2011, and was shot dead by police in Milan four days after the Dec. 19 attack in Berlin.

The searches focused in the capital and nearby Aprilia, where he was thought to have stayed after leaving a detention centre in Sicily in 2015, the source said. Police are investigating whether he was seeking to stay in Italy or trying to reach another country.

On Thursday, during an end-year news conference, Prime Minister Paolo Gentiloni said Amri was probably radicalised after arriving in Europe in 2011, but added that the government had no evidence Amri had “particular networks” in Italy.

“Five years ago he was not a jihadist ... In desperation, in isolation, in alienation, he found the conviction to follow the path of radicalisation,” Italy’s anti-terrorism chief Franco Roberti was also quoted as saying in an interview with la Repubblica newspaper on Thursday.

Roberti said lone-wolf attackers like Amri needed the help of small-scale criminal networks, such as those in Italy and Spain, for logistical support including acquiring false documents.

“From this point of view, Italy and Spain are the cradles,” Roberti said.

Amri’s undetected passage to Italy, via France, from Germany after the attack, has prompted eurosceptic parties to call for the reintroduction of border controls that were removed by the continent’s open-border Schengen pact.

Italy tried to deport him to Tunisia after he completed a four-year jail term for attempting to set fire to a building, but Tunisian authorities refused to take him, so he was released from the centre.

(This version of the story corrects name of Italian town in third paragraph to Aprilia from Acilia)