The arrest of two Romanian prostitutes in connection with a fatal attack on a young Italian woman in Rome last week triggered an immigration row on Monday.
Right-wing opposition lawmakers called for tighter immigration controls after the two prostitutes, aged 21 and 17, were arrested on Sunday.
The elder woman, named as mother-of-two Doina Matei, is accused of stabbing 23-year-old Vanessa Russo in the eye with an umbrella during a row last Thursday on the underground line at Rome's main Termini station.
The umbrella tip went through Russo's eye and severed a brain artery. She died in hospital a day later.
A police hunt was immediately launched for the two Romanians, who were caught on security cameras.
They were arrested in Tolentino in the central region of Marche on Sunday afternoon.
Matei told investigators on Monday that Russo's death had been a "tragic accident".
She said she had rowed with Russo while getting off the train and that the Italian had slapped her.
She said she had held up her umbrella in self-defence and that it had caught Russo in the eye.
But Matei's version did not square with eyewitness accounts or investigators' findings.
Investigating magistrates told reporters at a Monday press conference that Matei's reaction had been "wilful and excessive".
"She acted with power and force as the autopsy has shown... The gravity of the act was immediately clear and it is impossible to imagine that Matei was unaware of what she had done," they said.
"None of the numerous people who witnessed the incident have said that Matei was slapped. They have only said that they saw one of the Romanian women pushing Vanessa as she was getting off the train," they said.
Police sources said Matei had a police record, having been arrested for breaking immigration norms in November 2006.
They said she had been served with an expulsion order but Romania's entry into the European Union earlier this year had annulled the effect.
No charges as yet have been brought against the 17-year-old Romanian.
The opposition Northern League, a devolutionist party often accused of being anti-immigrant, said on Monday that the government was "morally and politically responsible for this tragedy" by having "failed to stem the flood of Romanian arrivals".
"It could have blocked our frontiers," the League said.
League Senator Piergiorgio Stiffoni said that "we sounded the Romanian alarm three months before Romania entered the EU... For the past ten years, these people from Eastern Europe, who have a reputation for being violent drunks, murderers and exploiters of women and children, have been coming to our country and committing crimes".
A lawmaker with the rightist National Alliance called for limits to be placed on the number of immigrants allowed to live in large Italian cities.
Welfare Minister Paolo Ferrero responded that "alleged immigrant emergencies have nothing to do with this case given that the two women arrested are Romanians and therefore European citizens".
"Right-wing politicians are only capable of Fascist-style ranting on the subject of immigration," the leftist minister added.
