Updated

The United States expressed deep concern and sadness Wednesday over the self-immolation of the mother of a prominent Vietnamese blogger and called on the communist government to free her daughter and two other writers facing trial.

A police officer in the southern province of Bac Lieu said Tuesday that 64-year-old Dang Thi Kim Lieng died Monday afternoon on the way to the hospital in Ho Chi Minh City after setting herself on fire that morning. Her death was not reported in state-controlled media.

Lieng's daughter, Ta Phong Tan, is a former police officer who wrote Cong ly va su that (Justice and Truth), a blog that addressed social justice issues. She was arrested last September on charges of conducting propaganda against the state.

The U.S. Embassy said in a statement that it was "deeply concerned and saddened" to hear of Lieng's death and it called on the Vietnamese government to release all three bloggers immediately.

Lawyer Ha Huy Son said Tuesday that Tan and two other bloggers are scheduled to be put on trial Aug. 7. All three belong to the outlawed Free Journalists Club and stand accused of posting and writing hundreds of articles that "distorted and opposed" the communist government.

If convicted, the three face up to 20 years in prison.

The other bloggers are Nguyen Van Hai, known as Dieu Cay, and Phan Thanh Hai, known as Anhbasg. They were arrested in 2008 and 2010, respectively, on the same charges as Tan, and in addition they stand accused of attending a nonviolence training course aimed at overthrowing the government.

Although Nguyen Van Hai finished serving a 30-month sentence for tax evasion in October 2010, he has not been released.

On a visit to Hanoi this month, U.S. Secretary of State Hillary Rodham Clinton mentioned the Free Journalists Club while saying she remains concerned about a lack of online freedom and the jailing of Vietnamese journalists, bloggers, lawyers and dissidents for peaceful expression.

Vietnam is a one-party state that does not tolerate challenges to its authority.