DUSHANBE (Reuters) - Tajik security services have detained the eldest son of a former elite police force commander who defected to Islamic State two years ago, suspecting that the young man was going to join his father, security sources said on Wednesday.

The United States last August offered up to $3 million for information about Colonel Gulmurod Khalimov, whom it trained in counter-terrorism before he joined Islamic State.

Washington has described Khalimov as “a key leader” of Islamic State, which has seized parts of Syria and Iran and staged or inspired deadly militant attacks around the world.

Bekhruz Khalimov, 18, Khalimov’s eldest son, was detained in late April when trying to fly from Dushanbe to Moscow, the security sources told Reuters. They said he remains under preliminary arrest, which can run up to two months under Tajik law.

Bekhruz Khalimov is the eldest of the eight children whom Gulmurod Khalimov left behind, along with his then-wife, when he defected in May 2015. He took with him his mistress and their four children, according to state prosecutors.

Thousands of people from the predominantly Muslim ex-Soviet region of Central Asia have joined Islamic State and several men have been detained this year over a bombing of the metro in Russia’s St Petersburg as well as a truck attack in Stockholm.