Italian police investigating the deadly attack on a Tunis museum in March said on Wednesday they had arrested a Moroccan man who arrived in Europe on an illegal migrant boat.

Majid Touil, 22, was detained on Tuesday evening in the northern Italian town of Gaggiano. He was known to have travelled to Italy in February on a migrant boat, and was living with his mother and two older brothers near Milan.

“A Moroccan national, wanted internationally, was arrested yesterday evening in a town in the Milan region. The Tunisian authorities suspect him of having taken part in the Bardo attack,” said a police spokesman.

It is not clear whether, or how, he returned to Tunisia for the terror attack in which 21 tourists died, but his arrest will raise fears of links between the Mediterranean human traffickers and Islamist groups.

Italian police said Touil was wanted for premeditated murder, kidnapping and terrorism, according to the police. Police seized papers and USB flash drives from his house.

Touil entered Italy illegally in February with a boatload of 90 migrants. He was then issued with an order to leave, said Bruno Megale, Italy’s anti-terrorism police chief, but it was not clear whether Touil had actually been expelled.

Touil’s brother told Italian news agency Ansa that he had not left Italy since February.

“My brother is innocent, he has not committed any crime. He arrived on a boat like many others, and from that moment he did not leave again,” he said.

Right-wing politicians seized upon the arrest to call for a crackdown on migrant arrivals.

Matteo Salvini, head of the anti-immigration Northern League party, called for the Schengen agreement allowing free movement between most continental European states to be suspended.

“Libyan intelligence says boats are arriving with Islamic State terrorists. Today in my Milan a North African was arrested for involvement in the Tunisian massacre. Close the borders before it’s too late,” said Mr Salvini.

Daniela Santanche, a hard-right politician from Silvio Berlusconi’s Go Italy party, also condemned the government.

It is “unbelievable that this government, instead of defending us from cutthroats, has transformed Italy into a useful platform for terrorists,” Ms Santanche said.

Matteo Renzi, Italy’s prime minister, thanked the police for capturing the suspect, but was inundated by demands to explain the affair to parliament. Angelino Alfano, the interior minister, faced calls to resign.

The attack on the Bardo National Museum on March 18 left 24 people dead: two gunmen, a Tunisian policeman and tourists from Italy, Japan, France, Spain, Colombia, Australia, Britain, Belgium, Poland and Russia.

Visitors getting off buses outside the museum were gunned down by two black-clad shooters with automatic weapons, who then took hostages inside the building.

Many people were shot in the back as they tried to escape. After rampaging through the museum for several hours, the gunmen were killed in an assault by security forces.

Tunisia has seen an upsurge in Islamic extremism since the overthrow of longtime strongman Zine El Abidine Ben Ali in 2011.

Authorities say as many as 3,000 Tunisians have gone to Iraq, Syria and Libya to join jihadist ranks, raising fears of returning militants plotting attacks.