A police officer frequently provided Proud Boys leader Enrique Tarrio with internal information about law enforcement operations in the weeks before other members of the far-right group stormed the US Capitol, according to messages shown at the trial of Tarrio and four associates.

In court in Washington DC on Wednesday, a federal prosecutor showed jurors a string of messages that Shane Lamond, a Metropolitan police lieutenant, exchanged with Tarrio in the run-up to the attack on the Capitol on 6 January 2021. Lamond, an intelligence officer, was responsible for monitoring groups like the Proud Boys.

On 6 January, supporters of Donald Trump stormed Congress in an attempt to block certification of Joe Biden’s election win. Nine deaths have been linked to the riot, including suicides among law enforcement

Less than three weeks before the riot, Lamond warned Tarrio that the FBI and Secret Service were “all spun up” over talk on an Infowars internet show that the Proud Boys planned to dress as Biden supporters on inauguration day.

A justice department prosecutor, Conor Mulroe, asked a government witness, the FBI special agent Peter Dubrowski, how common it was for law enforcement to disclose internal information in that fashion.

“I’ve never heard of it,” Dubrowski said.

Tarrio was arrested in Washington two days before the Capitol attack and charged with burning a Black Lives Matter banner taken from a historic Black church in December 2020. He was released and was not in Washington on 6 January.

In a message to Tarrio on 25 December 2020, Lamond said Metropolitan police investigators had asked him to identify Tarrio from a photograph. He warned Tarrio that police might be seeking a warrant for his arrest.

On the day of his arrest, Tarrio posted a message to other Proud Boys leaders that said: “The warrant was just signed.”

Before trial, Tarrio’s attorneys said Lamond’s testimony would be crucial, supporting Tarrio’s claims he was looking to avoid violence.

In court, Mulroe said Lamond asserted his fifth amendment privilege against self-incrimination. Tarrio’s attorneys have accused prosecutors of bullying Lamond into keeping quiet by warning the officer he could be charged with obstructing the investigation into Tarrio, a Miami resident who was the national chairman of the Proud Boys. Prosecutors deny that claim.

Tarrio’s attorney Sabino Jauregui said other messages showed that Tarrio cooperated with police and provided useful information. Jauregui said prosecutors “dragged [Lamond’s] name through the mud” and falsely insinuated he is a “dirty cop” who had an inappropriate relationship with Tarrio.

“That was their theme over and over again,” Jauregui told the presiding US district judge, Timothy Kelly.

Lamond was placed on administrative leave in February 2022, according to Mark Schamel, an attorney who said Lamond aided in Tarrio’s arrest for burning the banner. On Wednesday, Schamel said Lamond’s job required him to communicate with protesting groups and his conduct “was appropriate and always focused on the protection of the citizens of Washington DC”.

“At no time did Lt Lamond ever assist or support the hateful and divisive agenda of any of the various groups that came to DC to protest,” Schamel said. “More importantly, Lt Lamond is a decorated official who does not condone the hateful rhetoric or the illegal conduct on January 6 and was only communicating with these individuals because the mission required it.”

Tarrio and four lieutenants are charged with seditious conspiracy for what prosecutors say was a plot to stop the peaceful transfer of power.

Proud Boys members describe the group as a politically incorrect men’s club for “western chauvinists”. They often brawl with antifascist activists.

In a message to Tarrio on 18 December 2020, Lamond said other investigators had asked if the Proud Boys were racist. The officer said he told them the group had Black and Latino members, “so [it was] not a racist thing”.

“It’s not being investigated by the FBI, though. Just us,” Lamond added.

“Awesome,” Tarrio replied.

In another exchange, Lamond asked Tarrio if he called in a tip claiming responsibility for the banner burning.

“I did more than that,” Tarrio said. “It’s on my social media.”

In a message to Tarrio on 11 December 2020, Lamond told him about the whereabouts of antifascist activists. The officer asked Tarrio if he should share that information with uniformed officers or keep it to himself.

Two days later, Tarrio asked Lamond what the police department’s “general consensus” was about the Proud Boys.

“That’s too complicated for a text answer,” Lamond replied. “That’s an in-person conversation over a beer.”