Delphine Batho also criticised the dam which she said would serve a small minority.
It is unclear how a 21 year old activist died during a violent clash between demonstrators and police at the site of the contested dam project.
The general prosecutor of the nearby town of Albi, Claude Derens, told a news conference it was "impossible" to determine the cause of death of the 21-year-old activist during the unrest, which also left seven security personnel injured.
"We are not saying that the security forces killed a protester, but a witness told us the death happened at the time of the clashes," said Ben Lefetey, a spokesman for the organisers.
Lieutenant Colonel Sylvain Renier of the Tarn police said between 100 and 150 "anarchists" wearing hoods threw incendiary devices at the security forces at around 2:00 am.
The security forces used tear gas, stun grenades and flash balls against the demonstrators, Renier said.
Some 2,000 people had gathered to protest against the controversial Sivens dam project in the Tarn region.
Those who oppose the construction of the dam are concerned that an area of wetland which is home to 94 protected species, will be destroyed to make way for the dam. They also say the dam is too expensive and will only benefit a handful of agri-businesses.
The regional authorities who approved the project say a new wetland area will be created, and that the dam reservoir will be able to hold more than a million cubic metres of water, important for agriculture in the area.
The protesters included prominent politicians Jean-Luc Melenchon, leader of the Left Party, and alter-globalisation activist Jose Bove, co-leader of the European Greens.
Protests and clashes have intensified at the site since the start of clearing work early last month.