The FBI has infiltrated a militia-style group that one man involved in the pro-Trump insurrection at the US Capitol on 6 January formed under the guise of a Bible study group in Virginia.

Members who got involved discussed surveilling the Capitol building in Washington, DC, and their wish for secession from the US, and investigators closely followed one member’s plans to build and test molotov cocktails, according to recently unsealed court records.

Fi Duong, a northern Virginia man charged with joining the mob that stormed Capitol Hill on the urging of Donald Trump, told an undercover FBI agent that he belonged to a militia-style group and coordinated the “surveillance efforts”, although the reasons for doing so are unspecified.

An undercover agent attended meetings of Duong’s group and investigators intercepted encrypted communications about post-riot surveillance work, according to the court documents.

“How do we feel about an Intel run around the Capitol tonight?” an unnamed associate asked Duong during a 13 February exchange on the encrypted messaging platform, indicating purported intelligence-gathering efforts.

“Fewer of them out. Posture may be lowered. Good opportunity to expose weaknesses,” the message continued.

Duong responded, according to an FBI agent’s affidavit: “Poke and prod. But have a legitimate reason to go. Visit a restaurant or something. Get something cheap. Walk around a bit.”

The same associate was seen driving around the US Capitol after telling the group on 2 April that he planned to conduct surveillance on the building.

Duong is one of more than 520 federal defendants charged in the Capitol riot so far.

The FBI has linked dozens of them to far-right extremist groups, including several members of the Proud Boys and Oath Keepers who are charged with conspiring to mount a coordinated attack on 6 January.

Duong isn’t charged with plotting any violence, however, and prosecutors didn’t seek his pre-trial detention after he was arrested last Friday. He was freed after his initial court appearance.

Sabrina Shroff, an assistant federal public defender who represented Duong at the hearing, declined to comment on the government’s allegations.

A 30 June complaint charges Duong with disorderly conduct, obstruction of an official proceeding, and knowingly entering or remaining in any restricted building or grounds without lawful authority.

Duong held meetings at his home in Alexandria, a Washington suburb, the agent’s affidavit says.

The group called themselves “a Bible study” but also talked about firearms training events, the FBI said.

During a 12 February meeting, Duong discussed a Virginia secession movement that he said would be peaceful, according to the affidavit.

A man whom Duong described as a member of the Three Percenters far-right militia movement addressed a group meeting on 20 March, the affidavit said.

Duong said his loosely affiliated, unnamed group was similar to a militia based in northern Virginia but that his group’s mission was different because they “can’t be out in the open” and needed to be “a little bit more cloak and dagger”, the agent wrote.

Duong said he went to the 6 January rally in Washington alone and dressed in all black to look like a “member of Antifa”, referring to the anti-fascist activists who often gather at protests, the affidavit said.

Court papers don’t disclose Duong’s age. They say his aliases include “Monkey King” and “Jim”.