Islamic State has claimed the attack in Tajikistan which killed two American citizens.

The Tajikistan terrorist attack which on Sunday took the lives of Irish American Lauren Geoghegan, her partner Jay Austin, and two other cyclists has been claimed by the Islamic State (IS).

Geoghegan and Austin, both aged 29, had last year quit their jobs and set out to cycle the world but their journey was tragically cut short when they were knocked from their bikes and attacked with knives while journeying through Tajikistan.

The couple was members of a group of seven cyclists traveling north through the Danghara district, about 55 miles southeast of the Tajikistan capital Dushanbe. The group is said to have been struck by a car carrying five armed men who then left the vehicle to attack the cyclists with knives and a firearm.

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Lauren Geoghegan. Image: Simply Cycling.

Lauren Geoghegan. Image: Simply Cycling.

A Swiss man and a Dutch national were also killed in the attack, while the other three cyclists in the group were injured.  

While authorities investigated the possibility of a robbery or a hit and run, IS claimed the attack via a Twitter post on Monday. The post claimed that a "detachment from the soldiers of the Caliphate" had launched the attack on "citizens of Crusader coalition countries."

"We give glad tidings to the citizens of the Crusader coalition countries with what will disturb them, and what is coming is more devastating and bitter, Allah permitting,” it continued.

The State Department has declared the attack was “senseless,” while the US Embassy also condemned the attack.

Lauren Geoghegan and Jay Austin. Image: Simply Cycling.

Lauren Geoghegan and Jay Austin. Image: Simply Cycling.

"We strongly condemn the cruelty of the attackers and recognize that they in no way represent the kindness and hospitality of the Tajik people," the embassy said in a statement.

If a terrorist attack is confirmed, Geoghegan and Austin will be the first US citizens to be killed in a terrorist attack outside of the US since the attack in Barcelona in 2007.

It is believed that four of the suspects in the attack have been arrested while the fifth was “eliminated” while resisting arrest.

The attack comes during a “year of tourism” in Tajikistan, as they market the roads through their mountains and valleys to cyclists. As such, the interior minister Ramazon Rakhimzoda was cautious about labeling the deaths as a terrorist attack.

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Lauren Geoghegan. Image: Simply Cycling.

Lauren Geoghegan. Image: Simply Cycling.

"We are looking at versions that this was an accident, armed robbery, murder. The version of a terrorist act is also not being excluded," he said.

American victims Austin and Geoghegan had been on the road for just over a year, traveling together throughout Europe, Africa and Asia and chronicling their adventures on their blog and Instagram accounts.

"Badness exists, sure, but even that's quite rare. By and large, humans are kind,” Austin wrote back in April.

Lauren Geoghegan. Image: Simply Cycling.

Lauren Geoghegan. Image: Simply Cycling.

“Self-interested sometimes, myopic sometimes, but kind. Generous and wonderful and kind. No greater revelation has come from our journey than this."

The couple, Geoghegan from California and Austin from New York, had planned to travel together until their money ran out, living frugally and enjoying meeting with new friends on their travels. Although not always an easy journey, the couple said on their blog "we like to travel — and each other — a whole lot, and we want to do a lot more of it together. ... A bike ride around the world sounds like a hell of an adventure."

Lauren Geoghegan, her partner Jay Austin. Simply Cycling