Hunter Biden's former girlfriend Zoe Kestan testified that she saw him smoking crack as late as Sept. 20, 2018 -- a timeline that could undercut the defense's argument that he was abusing alcohol and not drugs when he purchased a gun on Oct. 12.

The defense has argued that Hunter Biden was not abusing drugs at the time of the gun purchase, after he had undergone a 12-day rehab stint in California that spanned from late August to early September. They argued repeatedly that when Hunter said in his audiobook that he "relapsed" following that rehab stay, it was only an alcohol relapse.

Kestan, however, testified that in mid-September he used crack "in the bedroom, in the bathroom."

Jurors have been laser-focused on Kestan as she testifies, some sitting up straight in their seats -- in contrast to the earlier testimony from the FBI agent who entered hundreds of text messages and audio book portions into the record, during which some jurors appeared to struggle to remain attentive.

Jurors listened attentively to the testimony of Hunter Biden's ex-girlfriend Zoe Kestan, turning their chairs to face her as she recounted their monthslong relationship in 2017-2018 during his addiction.

Under questioning, she spoke of multiple-night hotel stays, trips to Atlantic City, and memories of night clubs, dinners, and drug dealers.

Evidence displayed by prosecutors during her testimony included highly personal photos she had taken during their relationship, including a photo of her and Hunter Biden in a bathtub together in California, with a crack pipe in his hand.

As Kestan testified, Hunter's current wife Melissa sat in the front row of the gallery, barely moving or reacting.

Sitting at the defense table, Hunter Biden at times rubbed his face or covered his mouth as Kestan recounted their time together and photos flashed across the screen in the courtroom.

"I felt a connection with him," Kestan told the jury.

Prosecutors called Hunter Biden's ex-girlfriend Zoe Kestan to the stand, where she testified that she first met him at a Manhattan strip club in late 2017 and dated him for several months afterward.

Kestan bore witness to Hunter Biden's use of crack "every 20 minutes or so."

Kestan, who testified under subpoena and with an immunity agreement with prosecutors, recalled that Hunter Biden's demeanor didn't change after he used crack.

She described him as "cognizant and coherent."

"I remember thinking to myself that there was no change in his behavior," she said. "Nothing had changed, he was the same charming person."

Prosecutors showed jurors photos from Kestan's phone that showed crack pipes strewn about hotel rooms where they stayed.

On cross-examination by defense attorney Abbe Lowell, Hunter Biden's ex-wife Kathleen Buhle testified that she never personally saw Hunter Biden using drugs.

"Did you ever see Hunter using drugs?" Lowell asked.

"No," Buhle replied.

Buhle's answer prompted prosecutors to ask Buhle what she saw "if [she] didn't' see him using drugs?"

"He told me when I found the pipe what it was ... and because he was acting not himself," Buhle said.

Lowell also asked Buhle about Hunter Biden's alcohol use, to which Buhle testified that he abused alcohol for many years.

"He went to rehab the first time in 2003," Buhle said.

Attorneys for Hunter Biden, in their cross-examination of FBI Special Agent Erika Jensen, attempted to push back on prosecutors' contention that Hunter Biden was abusing drugs at the time he said on a gun-purchase form that he was drug-free.

A key piece of evidence comes from Hunter Biden's 2021 memoir, in which he described his "relapse" just weeks after leaving rehab in August -- just six weeks before purchasing the firearm.

"I stayed clean for two weeks then relapsed," Hunter Biden wrote in his book.

Defense attorney Abbe Lowell, on cross-examination, questioned the assumption that Hunter Biden's relapse implied that he was referring to his drug use -- suggesting instead that he was referring to his alcohol addiction.

"When he was referring to that in his book, was he talking about relapsing to drugs and alcohol, or do you know what he meant?" Lowell asked.

"I can only state what was stated," Jensen said.

"Which is, 'Then I relapsed?'" Lowell asked.

"Just what it says, 'I relapsed,'" Jensen replied.

Jensen also acknowledged that she could not verify that Hunter Biden was continuously using drugs from 2015 to 2019, saying, "My recollection is that there are excerpts where he was principally occupied with smoking crack cocaine. I didn't get the sense that it was the entire history."

"Meaning that there were periods of time that he was not?" Lowell asked.

"I think there -- including the period in August where we have some invoices for rehab, that there were periods where there was not," Jensen said.

It was an important moment for the defense, as Lowell managed to establish that Hunter Biden's use of drugs was not consistent -- cuing up his argument that the president's son was not on drugs at the time of his firearm purchase.

Prosecutors also entered into evidence records related to enormous cash withdrawals Hunter Biden made -- more than $150,000 from September through November of 2018, including a $5,000 withdrawal on the day he purchased the gun. Prosecutors suggested this cash was used to procure drugs; Lowell, on cross-examination, established that Hunter Biden paid $900 in cash for the weapon and accessories.

Court was subsequently recessed for the day, with Lowell's cross-examination of Jensen scheduled to resume Wednesday morning.

Prosecutors suggested that Hallie Biden would be the next witness after Jensen, making it likely that the first Biden family member to testify will take the stand at some point tomorrow.