International human rights groups praise China's release of a military doctor who was detained last month after he called for a review of the government's 1989 crackdown on pro-democracy demonstrators in Beijing's Tiananmen Square.

Dr. Jiang Yangyong alarmed the Beijing leadership with his letter to the authorities last February. In it, the 72-year-old military surgeon called for a review of the Tiananmen crackdown 15 years ago in which soldiers used tanks and guns against unarmed demonstrators calling for democracy and freedom of expression.

Hundreds, perhaps thousands, of mostly young university students were killed.

The government termed the demonstrations a "counter-revolutionary rebellion" and bans any public discussion of the crackdown, which occurred in the early hours of June 4, 1989. Every year leading up the anniversary, police detain relatives of the victims and activists who speak out against the killings. Dr. Jiang was detained on June 1.

The doctor was also in the spotlight last year, when he exposed the severity of the outbreak of the deadly SARS virus, Severe Acute Respiratory Syndrome, which the government tried to keep secret.

In his letter on the Tiananmen incident, Dr. Jiang said the government should declare the demonstrations a patriotic movement and he said the Chinese Communist Party should resolve what he described as the errors it had committed.

U.S. officials and Dr. Jiang's relatives said he was released in recent days.

While praising the Chinese government's decision to free the surgeon, human rights advocates say officials appear to be far from apologizing for the 1989 crackdown.

Many people here believe a review will eventually happen, but no one expects it to come soon. At a news conference earlier this year, Prime Minister Wen Jiabao repeated the party line that the crackdown was necessary for the sake of China's development. "We successfully stabilized the general situation of reform and opening up in China, and safeguarded the cause to build socialism with Chinese characteristics," he says. "Fifteen years have passed. During this time tremendous achievements were made in Chinese reform, opening up and socialist modernization."

Louise Vischer with the human rights group Amnesty International in London says advocates will continue to push Chinese leaders for a review of the incident. "If they are serious about opening up and they are serious about being more transparent, and improve their human rights record, it would be so important in this instance to just put and end to the discussion about the Tiananmen massacre in the hopes that for once there would be an inquiry," she said.

Amnesty says it has reports of at least 50 people - and perhaps many more - who remain in prison for their involvement in the Tiananmen demonstrations. Every year since the crackdown, Amnesty has launched appeals for their release.