Svetlana Davydova, 37, was arrested Jan 21 at her home in the town of Vyazma, west of Moscow. The Federal Security Service, the main KGB successor, had detained her on charges of treason that carry a prison sentence of up to 20 years.
Even though released, Davydova's case is still pending. Her youngest child is a 2-month-old baby whom she was breastfeeding.
Investigators haven't revealed details of the accusations, but her husband, Anatoly Gorlov, says Davydova called the Ukrainian Embassy after she had overheard a serviceman on a bus saying he and his comrades were preparing for a trip and decided he was talking about being sent to Ukraine.
"I think it happened thanks to a clear stance taken by the civil society and because of a strong support by independent media," Gorlov told The Associated Press.
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Despite mounting evidence, Moscow denies any role in fighting in eastern Ukraine between pro-Russian insurgents and Ukrainian government forces that has killed more than 5,300 people since April. When the Ukrainian army captured several Russian paratroopers in eastern Ukraine last summer, the Kremlin insisted the soldiers got lost.
Nearly 30,000 people have signed an online petition in support of Davydova, and activists picketed the FSB's headquarters in Moscow.
"Despite the gravity of the crime, this one circumstance can't justify keeping a mother of many children behind bars," he said.
Gorlov said he had been called in for questioning by senior regional secret service officers, but had refused to testify against his wife.
"I don't think she's guilty of treason. I think it's a totally political persecution," he said.