The U.S. Embassy in Tunisia said Wednesday it is "deeply troubled" that the 20 people convicted of  attacking the embassy last year only received suspended sentences.

A mob of some 2,000 people, mostly religious conservatives, attacked the compound on Sept., 14, 2012, destroying cars, property, burning the American flag and a nearby American school, ostensibly in response to a movie in the U.S. that allegedly insulted Islam. Four protesters died in the attack.

Only 20 people were prosecuted for the assault, and the court gave them two-year suspended sentences Tuesday night for attacking property and violating a state of emergency.

"We are deeply troubled by reports of suspended sentences," the embassy said.  "The verdicts do not correspond appropriately to the extent and severity of the damage and violence that took place."

The sternly worded statement said Tunisia's government must show there is no tolerance for those resorting to violence to achieve their goals and that the court's "decision fails in this regard."

The leader of the ultraconservative Ansar al-Shariah organization, Seifallah Ben Hassine, is being sought in connection with the attack, but he remains at large after slipping through a police cordon at the time.

The embassy called for an investigation and said those behind the attack should be brought to justice.

The violence was deeply embarrassing to the Tunisian government, which is run by moderate Islamists, and it marked the hardening of the state's position to the rising power of the ultraconservative Muslims known as salafis.

After tolerating them for two years as they became more aggressive in preaching their version of the religion and attacking aspects of society they disliked, government started cracking down on them.

On May 19, police prevented Ansar al-Shariah from holding its annual conference and insisted the salafis get permission before holding impromptu preaching sessions on the streets of Tunisia's cities.

Seventy-three people were arrested in connection with the attack on the embassy. Of those only 20 were prosecuted. The one-session trial was unusually brief for such cases.

Tunisians overthrew their long-ruling dictator in January 2011, sparking pro-democracy uprisings across the region. The transition, however, has been wracked by unrest and included the rise of extremist Islamist groups that have battled to increase religiosity in this country of 10 million people.