The assailants had been monitoring the property and may have observed that she often shares flowers with her neighbors, she said.
Omarov then sent those details to Mehdiyev, who lived in Yonkers, New York, prosecutors said.
Amirov and Omarov then arranged for Mehdiyev to get $30,000 in cash, which he used to buy an assault rifle and ammunition, prosecutors said.
Omarov, 38, was arrested in the Czech Republic earlier this month, and the United States is seeking his extradition.
Iran accuses Western powers of fomenting the unrest, which security forces have met with deadly violence.