On the night of Jan. 3, Peter Dahlin was on his way to catch a red-eye flight to Thailand.

He never took off.

In the days that followed, his colleagues at Chinese Urgent Action Working Group (China Action), an organization that trains and supports Chinese lawyers, pressed local authorities for information about his whereabouts but got nothing. The Swedish embassy asked to see him, but were denied contact, the group said.

On Wednesday, China Action went public with the news that Dahlin, 35, was detained on "suspicion of endangering state security."  A spokesperson for China's foreign ministry later confirmed the charge.

"Swedish citizen Peter Dahlin is suspected of engaging in actions that endanger China's national security," Hong Lei, a ministry spokesman, told the press. Dahlin will be treated "according to the law," he said.

China Action called the accusations "spurious" and demanded his release. "The Chinese authorities must immediately release Peter from detention and drop all charges against him," they said in a statement.

Dahlin first came to China as a student and worked in gender equality and sustainable development in Sweden before returning to to China in 2007, said Michael Caster, an American human rights researcher who is now serving as the group's spokesman.

Dahlin founded China Action to pursue his "passion to help Chinese civil society participate effectively in the development of the rule of law in China," Caster said.

The detention comes at a particularly sensitive time — for two reasons.

Crackdown on lawyers

China is now months into a sweeping assault on Chinese lawyers and legal advocates.

One of the Communist Party's current catch phrases is that China is a country "ruled by law." But since July 2015, the government has targeted more than 100 Chinese civil and human rights lawyers and legal advocates, detaining dozens and harassing their families, including one 16-year-old boy.

The crackdown casts some of China's most respected legal thinkers as enemies of the state. During Dahlin's nearly 10-day detention, authorities formally charged five with "subversion" and others with "inciting subversion," serious charges that could see them spend years in jail.

Those charged include Wang Yu, China's most prominent woman rights lawyer, and her husband, Bao Longjun, who have both spent months in detention. This fall, their 16-year-old son tried to flee the country, but was forcibly returned from Myanmar.

"It's a big surprise to all of us that they were charged with "subversion" which is even a stronger and more serious charge than [Nobel Peace Prize winner] Liu Xiaobo’s "inciting subversion" and could get them 10 years to life," said fellow lawyer Liu Xiaoyuan.

It is not yet clear if Dahlin worked directly with any of the detained lawyers. However, the group does the type of grassroots legal work that the state seems eager to stamp out: Training lawyers to take on rights cases, from demolition and eviction to arbitrary detention.

Caster said China Action suspected Dahlin was in trouble because police asked Chinese rights defenders about him during interrogations.

Draft NGO law

The detention also comes as China prepares to pass a new law to regulate foreign non-governmental organizations and their local partners.

The law, which emanates from President Xi Jinping's National Security Commission and was reportedly inspired by similar rules in Putin's Russia, puts foreign aid groups under the supervision of the China's vast security apparatus.

According to Chinese authorities, the law aims to protect the  “legitimate interests” of foreign NGOs while safeguarding China’s “national security and social stability.”

Though many countries have NGO laws, China's draft law has been widely criticized for treating non-profits as threats to Party rule instead of partners in development. Many fear it will be used to silence critics.

“The draft Foreign NGO Management Law seems to be based on the assumption that civil society organizations are serious threats to national security, and unfortunately, this sort of logic seems to be at play in this case as well," said William Nee, a Hong Kong based researcher for Amnesty International.

"This seems to be another case in which peaceful and legitimate work is being suppressed in the name of protecting national security.”

"You look at the draft NGO law and there’s definitely a concern that the Chinese government is probably looking to send a signal with Peter’s detention," said Caster.

“The message could be: 'See, we were right with the law we were working on.'”

Some now wonder if the case could presage the passage of the bill.

Late last year, China expelled Ursula Gauthier, a French correspondent, for an article that questioned the official line on terrorism.

The foreign ministry confirmed the move on Saturday, Dec. 26.  On Dec. 27, the country passed a new anti-terrorism law. And by Dec. 31 Gauthier was at the airport—and on her way home.

Xu Yangjingjing and Gu Jinglu reported from Beijing.