The son of a high-ranking Tajik official, whose conviction in Russia on drug-smuggling charges in 2010 sparked a spat between the two countries that is believed to have led to politically motivated arrests on both sides, has been cleared of wrongdoing.

A Moscow region court ruled on Dec. 28 that prosecutors did not present enough evidence to convict Rustam Khukumov of running a criminal gang that transported drugs across the Tajik-Russian border by train, the BBC Russian Service reported on Saturday.

Khukumov's father, Amonullo Khukumov, is head of Tajikistan's national railroad and a close ally of President Emomali Rakhmon.

Rustam Khukumov's 2008 arrest and subsequent sentencing to 9 1/2 years in prison became public knowledge only after Tajik authorities detained a Russian pilot and his Estonian counterpart in March 2011 when they stopped to refuel in Tajikistan after a NATO supply mission in Afghanistan.

The pilots received an 8 1/2-year prison sentence for smuggling and illegal border-crossing, sparking an outcry from the Russian government, which called the sentence "harsh" and "politically motivated," and said it "caused serious damage" to the two countries' relationship.

At the time, observers speculated that the arrest was an attempt by President Rakhmon to pressure Russia to release Khukumov, who is a dual Russian-Tajik citizen, or to possibly boost his nationalist credentials at home.

The tit-for-tat reached a boiling point in November, when police in Moscow and other cities launched an undeclared campaign against Tajik migrants, rounding up hundreds for deportation.

The Kremlin denied the connection between the roundups and the case, but insiders confirmed the link in numerous comments to the media. Specific nationalities have also been targeted during political crises in the past. Both pilots were freed later that month.

Khukumov's lawyer Konstantin Rybalov likewise said politics had nothing to do with the Moscow region court's decision. "If there had been irrefutable evidence of his guilt, it would have been impossible to release him. But that was never the case," he said in a statement on his blog.

About 1 million Tajik migrants work abroad, mostly as low-skilled laborers in Russia, where they earn remittances to send home to their families.