(CNN) -- A pro-democracy activist has been ordered detained in Vietnam for at least four months, having been charged with organizing terrorist activities, state media reported.
Nguyen Quoc Quan, a California resident who holds a U.S. passport, was arrested April 17 at Ho Chi Min City's Tan Son Nhat International Airport. But news of his arrest only became public Sunday, in reports from state-run news organizations and a plea from Viet Tan, the advocacy group for which he works.
According to a report in the Communist Youth Union-run Tuoi Tre news, "police caught (Quan) bringing documents on terrorist training to allegedly incite demonstrations in Ho Chi Minh City as well as other provinces and cities" during festivities set for Monday and Tuesday marking the reunification of North and South Vietnam and May Day.
Vietnam's Security Ministry determined that Quan planned to hold a "demonstration and terrorist activities planned by ... Viet Tan," said a report in the state-run Vietnam News Agency.
The Viet Tan group -- which has offices in California, Japan, Australia and France -- says on its website it is "committed to peaceful, nonviolent struggle" to defend human rights and promote democracy despite what it calls a "backward dictatorship."
Quan is being "temporarily detained for four months for further investigation" after having "admitted to his involvement in the criminal activities," according to the same VNA report.
His wife, Mai Huong, said Quan had gone to Vietnam planning to visit his younger sister and also talk about democracy and the rule of law.
"Whatever he does is nonviolent," she told CNN from Garden Grove, California. "How can the Vietnamese government do this to my husband?"
The government's terrorism accusations against the 58-year-old Quan are "completely fabricated and has no basis," added Viet Tan in its statement.
"The detention of Dr. Nguyen Quoc Quan is the latest example of the Vietnamese Communist Party's ongoing crackdown on human rights defenders," the group said.
A former high school teacher in Vietnam who earned a doctorate degree from North Carolina State University, Quan is "a long-time democracy activist and member of Viet Tan," according to the organization.
This is not the first time he's been detained in Vietnam.
He was released in 2008 after serving six months in prison for a terrorism conviction, according to Tuoi Tre news. His advocacy group, Viet Tan, said in that case he'd actually been detained in November 2007 "for distributing materials promoting nonviolent tactics for civil resistance."
Quan "continued his pro-democracy work" after his release, according to Viet Tan.
And before his latest trip, Quan got his paperwork approved by the Vietnamese government and believed he'd be picking up his visa at the airport, his wife said. When the couple spoke by phone, he told her he was heading to a hotel to take a shower -- until his phone abruptly cut off.
Mai Huong said she contacted U.S. consulate in Ho Chi Minh City on April 20, asking for help tracking him down.
Last Friday, a consular official called to tell her that Quan had been visited at a Ministry of Public Security detention center earlier that day.
Huong said she was told Quan appeared to be in good health and related a message, for her, not to worry and that he expected to be out in a few days.
But on Sunday, she learned that wouldn't be the case in light of the serious charges levied against Quan.
CNN's Brian Walker contributed to this report.