"I will leave (upcoming) electoral campaign arguments out of my statement: we are talking about the arrest of a terrorism suspect, about a successful investigation that was the result of good cooperation between several countries in the fight against terrorism," Angelino Alfano told parliament a day after Abdel Majid Touil, 22, was arrested on suspicion of involvement in the March attack in which 21 tourists died.
The case has triggered a furious reaction from opposition politicians after it emerged that Touil had arrived in Italy in February aboard a boat carrying hundreds of migrants.
He was photographed, finger-printed and given an order to leave the country within 15 days.
Authorities then lost track of him for the best part of three months until his arrest this week, which only happened because his mother, a legal resident of Italy, reported his passport missing.
Opponents of the centre-left government have seized on the case as evidence that jihadists can easily get into Italy by posing as migrants and that lax controls on arriving asylum-seekers mean they have no trouble staying.
Indirectly responding to demands for borders to be closed and for the expulsion of illegal immigrants to be accelerated, Alfano said the government had put in place proportionate and appropriate measures in relation to the arrival at southern ports of thousands of migrants every week.
"Given the scale and nature of the migrant phenomenon, it would be unreasonable to think our country is not exposed to this risk," he said.
"We are a great democracy and a great democracy faces these risks. The government is calmly and seriously dealing with them as best it can."
The minister recalled that security services were already on a state of heightened alert and revealed that Italy had expelled 33 terrorism suspects since the start of this year.
"It is a strategy that has allowed us to make arrests and to not suffer something dramatic," he added.
Alfano said there had been no basis for detaining Touil in February as there was no evidence at the time he had anything to do with jihadi activity.
Italian police only began looking for the suspect when the Tunisian authorities issued an arrest warrant for him and it was his mother's passport declaration in mid-April that enabled them to find him.
Touil's family insist he is not involved in terrorist activity. "When the attack happened in Tunis he was with me. My son has nothing to do with jihad or armed struggle," his mother told Italian media.
A local mayor in the part of northern Italy where the Moroccan was arrested said Touil did not appear to have the profile of a militant, saying he was attending adult education courses during the week of the Tunis attack.