"The most serious concern is caused by the information coming from the United States on the death of Russian citizen Igor Zyazin at the US Immigration and Customs Enforcement detention center in San Diego, the state of California," Dolgov said in a comment.

MOSCOW, May 4. /TASS/. Moscow wants the United States to investigate thoroughly all the circumstances of the death of Russian citizen Igor Zyazin, Russia’s Foreign Ministry Commissioner for Human Rights, Democracy and the Rule of Law Konstantin Dolgov said on Wednesday.

"As it is reported, he was taken into custody on April 24 this year while legally crossing the US-Mexican border," the comment said.

"We demand that the US notify the Russian side maximally promptly of all the circumstances of this tragic incidence and carefully investigate the death of the citizen of the Russian Federation and inform the Russian side in full of the results of the probe," Dolgov said.

As the comment says, "Russia’s Foreign Ministry keeps this issue under control."

In compliance with point 1, article 40 of the Russia-US consular convention of July 1, 1964, the US authorities are obliged to immediately notify of the arrest or another form of detention of a Russian national on its territory, Dolgov said.

"As we understand, this has not been done," the Russian diplomat said.

"What captures attention is that this is already the sixth death case at US Immigration and Customs Enforcement detention centers in 2016, which confirms the extremely unfavorable situation in US penitentiary institutions from the standpoint of the observance of human rights standards," Dolgov said.