A purported video shows the moment four Western cyclists were killed in Tajikistan on Sunday.
Received from an anonymous source and published by Radio Free Europe, it shows a car making a U-turn, knocking them down and driving over them on a narrow road in broad daylight.
After the vehicle ploughed into the tourists, the attackers then stabbed them.
The cyclists, who were from the United States, Switzerland and the Netherlands. were travelling through a rural area 90 km southeast of the capital Dushanbe.
It's the first known attack of its kind against Western tourists in Tajikistan, a remote ex-Soviet state north of Afghanistan in towering mountains where Islamists fought an insurgency against a Moscow-backed government in the 1990s.
On Monday, security forces killed four suspected attackers and detained one.
In a statement on Tuesday, the interior ministry said it had detained four more suspects and blamed the Islamic Renaissance Party of Tajikistan (IRPT) for the attack.
Citing what it said was the confession of a detained suspect, the ministry said the attackers' leader had been trained in Iran and the group planned to flee to Afghanistan after the attack.
"We completely deny the illogical allegation by the interior ministry and condemn this terrorist act," IRPT leader-in-exile, Muhiddin Kabiri said.
"This (statement) draws the attention away from the real criminals."
The Dushanbe government banned the IRPT in 2015, accusing it of plotting a failed coup.
Party leaders now in exile have denied those charges and said the move against them aimed to consolidate the grip of President Imomali Rakhmon, in power since 1992, the second longest-serving leader of an ex-Soviet state.
Kabiri said he was surprised that the government had ignored the claim made by Islamic State.
On Tuesday, Islamic State sought to bolster its claim of responsibility for the attack by releasing a video purportedly showing attackers pledging allegiance to the militant group.
Some of them appear similar to the suspects whose photographs have been published by Tajikistan's interior ministry.
Islamic State, which once controlled large parts of Syria and Iraq, is known to have had some presence in Persian-speaking Tajikistan: a former Tajik elite police force commander defected to the group in 2015.