Seventeen Tajiks have gone on trial on suspicion of involvement in the killing of four foreign cyclists on a southern mountain road in late July, according to Radio Liberty’s Tajik Service .

The trial started on October 23 and is being held behind closed doors inside Detention Center No. 1, a jail in Dushanbe.

Relatives of the defendants were not allowed into the building.

The four cyclists -- an American man and woman, a Dutchman, and a Swiss man -- were killed on July 29 when attackers plowed into their group on a road and then stabbed some of them.

Two other foreign cyclists survived the attack, which occurred about 150 kilometers south of the Central Asian country's capital.

Only one of the five men suspected of carrying out the attack, Hussein Abdusamadov, is among the defendants. The other four were killed by Tajik security forces.

The 16 other defendants are suspected either of involvement in organizing the attack or failure to report preparations for the attack.

The extremist group Islamic State (IS) claimed responsibility for the incident shortly after it occurred and released a video showing five men -- at least some of whom appeared to resemble those identified by Tajik officials as suspects killed in a confrontation with security forces -- pledging allegiance to the IS leader.

The Tajik government, however, rejected the claim and instead blamed followers of the Islamic Renaissance Party of Tajikistan (IRPT), a political party that was banned in Tajikistan in 2015.

The leadership of the IRPT -- which served for several years in the Tajik government -- has denied involvement and called the authorities' claims "shameless and illogical slander."