The alleged leader of the far-right Oath Keepers militia group, who is accused of taking part in the attack on the US Capitol, discussed the possibility of getting a boat to ferry “heavy weapons” across the Potomac River, prosecutors have said.

Court papers filed in the case against Thomas Caldwell provided new evidence in the plot on Capitol Hill as Democrats prosecuting Donald Trump’s impeachment made their case to convict the former president for inciting the violence.

The details come days after Caldwell’s lawyer said the man – who authorities said holds a leadership position in the extremist group – had a top-secret security clearance for decades and previously worked for the FBI.

Prosecutors said Caldwell sent a text message to someone believed to be affiliated with the Three Percenters, an anti-government movement, on 3 January about the possibility of sending weapons across the river.

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“How many people either in the militia or not (who are still supportive of our efforts to save the Republic) have a boat on a trailer that could handle a Potomac crossing?” Caldwell wrote, according to prosecutors.

“If we had someone standing by at a dock ramp (one near the Pentagon for sure) we could have our Quick Response Team with the heavy weapons standing by, quickly load them and ferry them across the river to our waiting arms.”

Prosecutors revealed the evidence to make the case that Caldwell should remain locked up while he awaits trial.

Authorities also said that during a search of Caldwell’s home, they found a “death list” that included the name of an elected official from another state.

Investigators also found invoices for more than $750 (£540) worth of live ammunition and what appeared to be a gun designed to look like a mobile phone, prosecutors said.

Caldwell’s lawyer is urging the judge to release him, saying he denies being a member of the Oath Keepers or ever going into the Capitol building.

The Virginia man is among more than 200 people charged with federal crimes so far in the deadly siege.

He was charged with conspiracy last month alongside two other alleged members of the Oath Keepers, who are accused of planning in advance to carry out violence.

Jessica Watkins, who authorities say conspired with Caldwell, indicated as Biden’s inauguration approached that she “was awaiting direction from President Trump”, prosecutors said in another court.

“I am concerned this is an elaborate trap,” Watkins wrote in a text message days after the election, according to the court papers.

“Unless the POTUS himself activates us, it’s not legit. The POTUS has the right to activate units too. If Trump asks me to come, I will.”