Thirty-four-year-old Serou's body was found on Saturday.
"In a auto with a stranger".
Popov, who is reported to have a record of violent crimes, could face life in prison if convicted.
Beccy Serou, the victim's mother, told NPR on Saturday that the last text she received from her daughter was regarding the ride she took with Popov. Marine has turned up dead in Russian Federation after catching a ride with a stranger and nervously texting her mom, "I hope I'm not being abducted, " according to reports.
She lived just outside the city, which is 420km (260 miles) east of Moscow, and texted her mother just after 18:30 local time (15:30 GMT) on Tuesday.
The body of Catherine Serou, a 34-year-old law student and veteran from California, was found Saturday morning.
A suspect with prior convictions is in custody, though he has not been named.
The U.S. Embassy in Moscow revealed it was "closely monitoring" the investigation and is giving "all appropriate assistance" to Serou's family in America.
"And that's the last thing she wrote", said Beccy Serou, who added she was hopeful for her safe return because she was a former US Marine who had done a tour in Afghanistan, according to NPR.
On Saturday, the Investigative Committee, which probes major crimes in Russian Federation, said it found the "missing foreign student".
Ms Serou had planned to return to the U.S. to work as an immigration lawyer and had been enjoying her time in Russian Federation, her mother told NPR.
Beccy Serou told NPR she believed her daughter may have gotten into a stranger's vehicle on accident instead of an Uber because she was in a rush to get back to a clinic where a payment she made did not go through.
She had planned on returning to the United States to pursue a career as an immigration lawyer.