U.S. officials had shown her and her husband "pictures of dead body of Tamerlan," she said at the news conference. "I did not look. I could not believe it is my son."
The shoplifting arrest isn't the first time Zubeidat Tsarnaev has been eyed by U.S. authorities.
Russia raised concerns to U.S. authorities about her in 2011 at the same time they asked the United States about her son Tamerlan, several sources told CNN.
U.S. authorities added both the mother and son to the Terrorist Identities Datamart Environment, or TIDE, database -- a collection of more than a half million names maintained by the National Counterterrorism Center, an intelligence official said.
FBI agents interviewed Zubeidat Tsarnaev as part of its investigation into her son, whose case was closed after several months.
'Because they were Muslim'
On Wednesday, FBI agents were in Makhachkala -- a city that Tamerlan Tsarnaev called home for several months in 2012 -- to talk with the suspects' parents.
The conversation, which included members of Russia's federal security service, ended Wednesday evening, the suspects' mother told Saratova.
Both parents have publicly said they believe their children are innocent and were framed: "just because they were Muslim," as Zubeidat Tsarnaev put it.
When asked whether she thinks her younger son will get a fair trial, she replied, "Only Allah will know."
The Tsarnaevs are originally from the troubled Russian republic of Chechnya but fled from the brutal wars there in the 1990s. The two brothers were born in Kyrgyzstan and then lived in Dagestan and moved at different times to the United States.
The family's adopted republic has become a focus for investigators, especially given that Tamerlan Tsarnaev went there during a six-month trip to Russia last year.
On two occasions before that -- in March and late September 2011 -- Russian authorities asked U.S authorities to investigate Tamerlan Tsarnaev.
Zubeidat Tsarnaev said the FBI had visited her family "several times" in 2011 with questions about Tamerlan's "Islamic interests."
"They said that they ... just think that Tamerlan is a kind of ... little on radical side of Islam and they just don't want ... they are keeping their eye on, you know, the boys, like young boys like Tamerlan so any bombing any like explosion won't happen in America. On the streets, like on the streets."
The FBI investigators told her there was nothing wrong with that, they just wanted everyone to be safe, she said.
The Tsarnaevs and Misha
A friend named Misha, whom Tamerlan met in the United States, steered the older suspect toward a more devout practice of Islam, Tamerlan's relatives have said.
"Tamerlan was close friends with him, so they think that Misha made him ... become more deeply religious," Zubeidat said.
She was impressed with the Armenian convert to Islam, who seemed "very intelligent, nothing wrong." He suggested that she cover her hair with a scarf, which she did.
"When Misha visited us ... he just opened our eyes, you know ... really wide about Islam. He was really, he's devoted, and he's very good, very nice man," Zubeidat Tsarnaev said.
Tamerlan's uncle Ruslan Tsarni had a less favorable opinion.
"This person just took his brain," he said. "He just brainwashed him completely."
Tamerlan, a former Golden Gloves boxer, left the ring and stopped listening to music under Misha's influence.
Elmirza Khozhgov, a former brother-in-law of the brothers, said the elder Tsarnaev brother introduced him to a man named Misha, but "I didn't witness him making him radical."