BANGKOK — The army detained two women and confiscated black T-shirts last week as part of an investigation into an underground group seeking to install a federal republic in Thailand, a defense spokesman said Monday.

Kongcheep Tantravanich said the authorities have been monitoring the group which has urged its supporters to wear black T-shirts with a red and white logo.

“The shirts belonged to the Organization For a Thai Federation,” Maj. Gen. Kongcheep said in an interview, using the group’s formal name. “We have to find out where they got the T-shirts from, and who made the T-shirts … We are investigating the matter.”

Read: Soldiers Raid Homes For Black Shirts, Abduct 2 Women

The republican activists confirmed the shirts as their own and denounced the arrests as an attempt to intimidate the public.

In a statement posted online, the organization said it had urged its supporters to don black T-shirts with the flag as a show of strength. The statement, which cannot be published in full due to legal concerns, went on to accuse the military of harassing its sympathizers in an effort to stamp out growing support for its cause.

Soldiers detained the two women in two separate raids at their home Thursday and brought them to the 11th Army Circle base for questioning. While the first woman, named Surangkanang, was released without charge on the same day, the other, identified as Wannapha by her family, remained in custody Monday.

Kongcheep said the two recently bought T-shirts with the same emblem used by the republicans.

“Their flag is right there on the shirts,” the spokesman said, adding that the group aims to secede parts of Thailand and establish a federal republic.

Another senior army official confirmed in a separate interview that the arrest and ongoing detention of Wannapha is part of a crackdown on the republican group.

“They might have links to Organization for a Thai Federation. This is a big issue,” said the official, who spoke on condition of anonymity because he’s not allowed to talk to the media. “Social media is distorting the news that it was only about the black T-shirts. But we have evidence that it goes deeper than that.”

Wannapha had no access to legal counsel throughout her detention since Thursday, civil rights activists said. Soldiers at the base, which has been converted into a special prison, only agreed to let Wannapha’s two young daughters visit her on Sunday.

The Organization for Thai Federation is reportedly run by a group of Redshirt activists living in exile in Laos. It’s difficult to gauge how much or little support it has, though its online videos typically rack up tens of thousands of views.

Though they occupy a niche in the spectrum of Redshirt politics, the group issues prolific dispatches, having published numerous videos on YouTube – most of them several hours long – in which they discuss their republican ideas between bouts of dizzying conspiracy theories.

In one such video published mid-August, the group leader, known as Uncle Sanam Luang, called upon his supporters in Thailand to don black T-shirts with their red and white flag emblem to raise awareness of the group.

“We have to make it a reality. Who would do it? The people. The people are owners of the country, not anyone else,” Uncle Sanam Luang says in the video. “Therefore, we have to wake up. We have to do everything we can.”

Secession is forbidden in the first section of the Constitution, while organized attempts to overthrow the monarchy are outlawed by Section 113 of the Penal Code. Violation of the latter is punishable by death.

“If we don’t take action, we’d end up being charged with negligence,” spokesman Kongcheep said. “It’s a duty for us in the security forces.”

Correction: The original version of this article incorrectly cited Section 116 as the law that bans organized attempts to overthrow the monarchy. It is in fact Section 113.

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