A probe into the cause of a French eco-protester's death in violent clashes with security forces veered towards riot police Tuesday, in a new development to a case that has shocked the country.

Remi Fraisse, 21, was killed in the early hours of Sunday as people protesting against a controversial dam project in the southwestern Tarn region fought with security forces. It was the first such death in mainland France in nearly three decades.

After an initial autopsy revealed Fraisse likely died from an explosion, forensic tests carried out on skin samples, remnants of Fraisse's rucksack and debris on the Sivens site found traces of TNT on his clothes.

"TNT is part of the composition of the (explosive) charges of tear gas or concussion grenades used by gendarmes," Claude Derens, the prosecutor in charge of the case, told reporters.

Gendarmes are security forces that come under the jurisdiction of both the defence and interior ministries and carry out police duties including riot policing.

Concussion grenades rely for their effect on the blast of their detonation rather than the fragmentation of their casing, and are designed to stun rather than kill.

"We cannot today exclude the role of a concussion grenade thrown from the enclosure where gendarmes had entrenched themselves overnight Saturday," Derens added.

However, questions remain over how the wound that killed Fraisse was found on his back when he was facing police.

Interior Minister Bernard Cazeneuve said Tuesday he had immediately suspended the use of concussion grenades by riot police until the probe determined exactly who was at fault in the tragedy.

Cazeneuve in response to a question whether it was an error, said: "No it wasn't a blunder. We mustn't portray it like that."

"The events are nothing like the other," he said, referring to Malik Oussekine, a 21-year-old French-Algerian who died in police brutality during a student protest in Paris in December 1986.

The head of the local assembly of the Tarn region will seek a suspension of construction of the dam until further notice, a senator told Europe 1 radio.

Fraisse's death sparked anger across the country, prompting President Francois Hollande to call Tuesday for "calm and understanding" as he scrambled to contain the fall-out from the tragedy.

Authorities also warily eyed a separate eco-protest in the northern city of Amiens Tuesday, where at least 1,500 people from all over the country converged in support of activists being tried for vandalising the site of an industrial super-farm.

- Big and violent group -

Arie Alimi, the lawyer for Fraisse's family, told French radio he suspected police were responsible for his death, which was being variously blamed on grenades used by security forces or on Molotov cocktails thrown by protesters.

"What I know is that there are no burns (on the victim's body). A Molotov cocktail causes burns," he said.

Jose Bove -- a Green MEP and activist who was at Saturday's protest that started peacefully but later spiralled out of control -- said witness statements also pointed to police responsibility.

He accused authorities of having "deliberately" resorted to the use of riot police in an empty area where there was no property or machinery to protect, "purely to create tension".

The victim's family have announced they will make two official complaints -- one for murder and the other for "violence that caused death without the intention to kill".

But Derens earlier defended police who he said were being mobbed by a hardcore group of protesters at the time of Fraisse's death.

They were "in front of an extremely big and violent group" that included the victim, he told RTL radio.

Prime Minister Manuel Valls also came to their defence, telling parliament he would not accept people calling into question the acts of those "who counted numerous injuries in their ranks" and are "often confronted with extreme violence".

The incident is the culmination of weeks of protests by those opposed to a dam they say will destroy a reservoir of biodiversity and will only benefit a small number of farmers.

Those promoting the project, including local politicians, retort that the dam is in the public interest as it will ensure irrigation and the development of high-value crops.