Tajik authorities say they have arrested five men in connection with the July 29 attack that killed four foreign cyclists, as Dushanbe widened its investigation.
Authorities said they had also ordered 20 Tajik migrant workers in Russia to return home for questioning.
Tajik police said on August 16 that the five men were arrested last week in the southern town of Norak, the hometown of two of the suspects in the attack.
Abdurahmon Gulov, a local official in Norak, said those arrested were Farhod Davlatov and his father Abdumumin Davlatov, and Bakhtovar Salohiddinov, Qiyomiddin Qurbonov, and Bakhtiyor Solehov.
Gulov said the five men were in contact via WhatsApp with Asliddin and Jafar Yusupov, the two brothers from Norak who were among four suspects killed in a police operation on July 29-30.
During the same operation, police detained Hussein Abdusamadov, the alleged cell leader of a terrorist group that carried out the July attack, the deadliest on Western tourists in Central Asian history.
Gulov said the Yusupov brothers encouraged the five men to join their terrorist cell and disclosed their plans to carry out terrorist attacks in Tajikistan.
Gulov said the five men were charged with failing to notify authorities of a possible crime.
Gulov said the 20 Tajik citizens ordered to return from Russia for questioning were in contact with the five men suspected of carrying out the attack.
Abdusamadov was a migrant worker in Russia before his return to Dushanbe around three months ago.
In the brutal July 29 attack on the foreigners, a car rammed into the group of cyclists before multiple attackers emerged from the vehicle and stabbed survivors, killing two Americans, a Swiss, and a Dutch national.
Three other foreigners were injured in the attack before the assailants sped off.
Islamic Party Blamed
The extremist group Islamic State (IS) claimed responsibility for the attack and released a video showing five men -- at least some of whom appeared to resemble four men identified by Tajik officials as suspects killed in a confrontation with security forces -- pledging allegiance to IS's leader.
But the government maintained the five men were members of the banned Islamic Renaissance Party of Tajikistan (IRPT), which the Tajik authorities have blamed for the attack.
The leadership of the IRPT -- which served for several years in the Tajik government -- has called the authorities' claims "baseless and irrational," as well as "shameless and illogical slander."
The IRPT was Central Asia's only registered Islamic political party until the Tajik government banned it in 2015. It had tens of thousands of members when it was outlawed and has since been labeled a "terrorist" organization by Dushanbe.