In Kasserine's graveyard two new tombstones have been added to the roll of people who have died a violent death in this Tunisian city close to the Algerian border. Ahlem Dalhoumi, 22, and her cousin Ons Dalhoumi, 18, were both killed with bullets to the head the night of August 23 on their way home from a party with five other family members.

At about midnight, survivors told me, their car was rattling over a decrepit roadway filled with potholes. As they neared the city, men in black emerged from behind the reeds and tried to halt the car. The driver, believing they were thieves, kept going.

Some yards further she heard gunfire, and stopped. But it was too late. The men in black who were, in fact, security force members, had already shot and killed her sister and cousin.

The two cousins had a packed schedule for the following day, starting school after summer vacation: Ahlem was going back to Germany, where she was born, and was studying law. Ons had just graduated from secondary school and was starting college in Sousse.

The police officers, in their statements, said the car approached them at high speed and did not stop, despite warning shots. They said they believed they were encountering terrorists. Numerous attacks in the area by armed groups have claimed 30 victims among the police and armed forces. The government has announced both judicial and internal administrative investigations.

When we visited the gravesite, Ahlem's father showed us another a few yards away. It belongs to Slah Dachraoui, a 19-year-old fruit vendor. He was the first person to fall under police fire in Kasserine during the popular uprising in 2011 against the regime of Zine el Abidine Ben Ali.

"If the government had gone after the one who killed him, maybe my daughter wouldn't have had the same fate," he said. Slah, like so many other young men, had decided to go out and show he was angry, fed up, wanted a better future.

Three years later the identity of those who killed more than 15 people and wounded more than 100 others between January 8 and 10, 2011 in Kasserine is still unknown. At her home in the working-class neighborhood of Hay Ennour, Slah's mother, Ribah Briki, also made the same connection, unprompted, between the two cases: "We still don't know who killed my son. Maybe if we had found out, these two young women wouldn't have been killed."

At bottom, these grieving parents' questions are about police impunity.

Many things have changed in Tunisia since the revolution. There is greater room for freedom. Everybody can express their opinions. Political pluralism and media diversity are firmly rooted realities, and legal and police reform have begun.

But there is nevertheless one constant that hasn't altered one iota: an absence of will to prosecute and try those responsible for police violence, be it due to excessive force during demonstrations, blunders, mistreatment, or even torture.

In most of these cases the truth is irrecoverable because the courts cannot disentangle the web of events and the chain of command leading up to these acts of violence.

The verdict for the three-year-long trial in the military courts, for the killing of protesters during Tunisia's uprising, announced in April, dealt a guillotine blow to any dreams of justice: three years of prison for those at the highest level of command, already free because they had spent that period in pre-trial detention.

In Kasserine only one police officer - the Nour district chief of police identified as the killer in two cases - was convicted. He got 2 years in prison on appeal, although the district court had sentenced him to 15.

The other cases had similar outcomes. A trial for use of birdshot against demonstrators in Siliana on October 12, 2012 has not gone forward although the victims have filed a complaint. Five protesters lost their eyesight as a result of the birdshot. Likewise, most of the cases dealing with torture and mistreatment during the former regime and after it fell have become stalled at one level or another.

The roots of this phenomenon are complex: a psychology of abusive police practices, a culture of violence that is resistant to change, and the absence of serious reform of the judicial system. Moreover, some police unions established after the revolution are inclined to rebel against any and all attempts to prosecute their colleagues, reinforcing "tribal" instincts.

In addition, the pervasiveness of the security rhetoric that envelopes the fight against terrorism relegates charges of abuse to the sidelines. Impunity fuels the cycle of violence: law enforcement officials who know they are protected and shielded by their institution and their unions, and who enjoy near-total immunity, feel they are invulnerable.

To be sure, the murders during the revolution and this most recent police blunder differ in context and scope. But they both raise the issue of how police use force, frequently with no consideration for human life or citizens' rights. The absence of a clear will to prosecute and try those who commit these kinds of human rights violations fosters an increasing fear that repressive practices, once thought abolished, will return, and that abuses will multiply.

The continuity Ahlem's father and Slah's mother spoke of between the killing of demonstrators at the time of the revolution and the murder of the two young women is a sort of vicious circle, which will only be broken if security forces and the courts make a genuine effort to objectively investigate and try those responsible, wherever the facts may lead. Major steps that could help include a code of conduct for police and clear guidelines for conditions and methods for the use of force, along with creating an independent enitity responsible for police oversight.

Amna Guellali is the Tunisia researcher at Human Rights Watch. Follow on Twitter @aguellali