WASHINGTON (AP) — An off-duty police officer stormed the U.S. Capitol because he believed the 2020 presidential election was stolen from Donald Trump and he wanted to interfere with the certification of President Joe Biden’s electoral victory, a federal prosecutor said Tuesday at the start of the Virginia man’s trial.

But a defense attorney told jurors that former Rocky Mount, Virginia, police officer Thomas Robertson only went into the Capitol because he wanted to retrieve a fellow officer who had entered the building before him during the riot on Jan. 6, 2021.

Jacob Fracker, the other off-duty Rocky Mount police officer who entered the Capitol that day, could be a key witness for prosecutors at Robertson’s trial. Robertson was a mentor and a father figure to Fracker, attorneys said during their opening statements.

Two other Capitol riot defendants already have been tried on federal charges arising from the Jan. 6 siege. The first two trials both ended with convictions, although a judge acquitted one of those defendants of a disorderly conduct charge.

Another trial for a Capitol riot case started Tuesday. While jurors heard testimony for Robertson’s trial, U.S. District Judge Trevor McFadden heard testimony without a jury for the case against Matthew Martin, who has worked for a government contractor at the National Laboratory in Los Alamos, New Mexico.

Martin, who is accused of remaining inside the Capitol for about 10 minutes, testified that he saw a police officer wave him into the building. He also said he followed the crowd into the building.

“I went with the flow,” said Martin, whose trial is scheduled to resume on Wednesday.

Fracker was set to be tried alongside Robertson this week, but he pleaded guilty last month to a riot-related conspiracy charge and agreed to cooperate with federal authorities.

Assistant U.S. Attorney Elizabeth Aloi said Robertson was armed with a large wooden stick and wearing a gas mask when he and Fracker joined the mob that overwhelmed police officers and breached the Capitol.

“His intent was to interfere with the election because it did not have the result that he wanted,” Aloi said.

Defense attorney Camille Wagner said Robertson, whom she called “T.J.,” knew that he had entered restricted areas of the Capitol where he wasn’t supposed to be on Jan. 6. But he isn’t accused of engaging in any violence or property destruction, she noted.

“All T.J. did was enter, retrieve, depart,” Wagner said.

Robertson used a large wooden stick to impede police officers who were trying to hold off the mob, according to prosecutors. Police body camera video captured his interaction with police.

Wagner said Robertson didn’t wield the stick as a weapon. She said the U.S. Army veteran was using it as a walking stick because he still has a limp from getting shot in the right thigh while working as a private contractor for the U.S. Defense Department in Afghanistan in 2011.

Robertson is charged with six counts, including obstruction of an official proceeding, civil disorder, entering and remaining in a restricted building while using a dangerous weapon and disorderly conduct in a Capitol building. Five of the counts relate to his actions on Jan. 6. The sixth stems from his alleged post-riot destruction of cellphones belonging to him and Fracker.

Fracker pleaded guilty to conspiring to obstruct an official proceeding, the joint session of Congress to certify the Electoral College vote.

Aloi said Fracker is ashamed of his conduct at the Capitol and is “not the same person today that he was on Jan. 6.”

Robertson and Fracker both served as police officers in Rocky Mount. The town, which is about 25 miles south of Roanoke and has roughly 5,000 residents, fired both of them after their arrests.

“He held a position of public trust,” Aloi said. “He broke that public trust when he participated in the attack at the Capitol.”

Robertson and Fracker drove with a neighbor to Washington on the morning of Jan. 6. Robertson brought three gas masks for them to use, according to prosecutors.

After listening to speeches near the Washington Monument, Fracker, Robertson and the neighbor walked toward the Capitol, donned the gas masks and joined the growing mob, prosecutors said. Robertson stopped to help his neighbor, who was having trouble breathing. Fracker broke off and entered the building before Robertson, but they reunited inside the Capitol.

Aloi showed jurors some of Robertson’s vitriolic posts on social media before and after the Capitol riot. In a Facebook post on Nov. 7, 2020, Robertson said “being disenfranchised by fraud is my hard line.”

“I’ve spent most of my adult life fighting a counter insurgency. (I’m) about to become part of one, and a very effective one,” he wrote.

Robertson was not “charged for his beliefs,” Aloi said.

“He was charged for his actions,” she told jurors.

Wagner said Robertson should be judged by his actions, not his words.

“We ask you to remember that actions speak louder than words,” she told jurors.

A Capitol police officer was the first witness to testify at Robertson’s trial. Capt. Ronald Ortega said the mob “severely outnumbered” officers who were trying to hold back the crowd.

“It just seemed unreal at the time,” Ortega said.

Robertson has been jailed since U.S. District Judge Christopher Cooper ruled in July that he violated the terms of his pretrial release by possessing firearms.

On March 8, a jury decided the first Capitol riot trial by convicting a Texas man, Guy Reffitt, of storming the Capitol with a holstered handgun. In the second trial, the same judge hearing testimony on Tuesday in Martin’s case convicted New Mexico county official Couy Griffin of illegally entering restricted U.S. Capitol grounds but acquitted him of engaging in disorderly conduct.

Reffitt and Griffin entered restricted areas outside the Capitol but not the building itself.