Secular opposition politician Mohamed Brahmi was killed with the same gun that was used to kill his coalition party leader Chokri Belaid six months ago, Tunisia's interior minister has said.
Lotfi Ben Jeddou told a news conference on Friday that it suggested the involvement of the same hardline Salafist group.
"The same 9mm automatic weapon that killed Belaid also killed Brahmi," Jeddou said.
He named the main suspect as hardline Salafist Boubacar Hakim, already being sought on suspicion of smuggling weapons from Libya.
Ben Jeddou said that ballistic examination of the bullets fired on Thursday at Brahmi showed they came from the same gun used to kill leftist Belaid in February.
Two men on a motorcycle shot Brahmi 14 times in front of his home as he was getting in his car.
Belaid was killed in a similar fashion.
The assassination has plunged the country into a political crisis, with the opposition calling for the dissolution of the government. A demonstrator was killed in the central Tunisian town of Gafsa overnight on Friday during a march protesting against the assassination, the AFP news agency reported.
Mohamed Moufli, 45, was killed as police used tear gas to break up the night-time march. He was struck in the head by a tear gas cannister and died soon afterwards in hospital.
Police acted as hundreds of demonstrators tried to storm local government offices in the town centre.
Tunisian trade unions called a nationwide general strike on Friday over Brahmi's murder.
Shops an banks were closed and the country's national airline Tunisair cancelled all flights on Friday, as the General Union of Tunisian Labour (UGTT) called for the nationwide strike in protest against "terrorism, violence and murders".
Protesters were again taking to the streets on Friday, after thousands of demonstrators rallied around the country on Thursday, with many holding the ruling Ennahda Party responsible for the assassination and attacking local party headquarters.
Ennahda, a moderate Islamist group, denied accusations that it was involved in the murder of a member of the People Movement Party.
Police used tear gas in central Tunis and in the provinces to disperse the protests.