Police in the northern port city of Tianjin said Zhao Wei, 24, was released after she admitted her crimes and showed a good attitude.
Zhao worked for prominent human rights lawyer Li Heping, and is the youngest among dozens of rights lawyers, workers and activists who were taken into custody last July in a campaign to curb growing rights activism spearheaded by legal workers.
She became a point of public focus because of her age, especially after she was formally arrested in January on the severe charge of state subversion and became one of China's youngest political prisoners. If convicted on the charge, she could face life imprisonment.
Li and at least eight other rights lawyers remain in custody.
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Today, Zhao's personal microblog was updated with a statement expressing gratitude to the police. Tianjin police later reposted the statement.
"The afternoon sunshine is so good," the statement said. "It's a good feeling to breathe freely."
Ren, who could not reach Zhao or her mother, said Zhao could have posted the statement herself but it might not reflect her true thoughts.
"The authorities are known for their means, and who can still stand up after one year?" Ren said, adding that police have never released any details of why Zhao was charged with state subversion.