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.

A juror in Hunter Biden's gun trial told ABC News that he did not know who Hunter Biden was before the trial started, and that his proximity to power made no difference to the jury.

"Everyone is human and everyone makes mistakes," the juror said. "And those mistakes can have serious consequences -- not just for them, but for everyone else."

The juror -- whose father was killed by gunfire when he was young -- said prosecutors successfully showed that Hunter Biden's drug use around the time of his gun purchase was "too close for me to consider him not a user or addict at that point."

"There's too many patterns that I can see that showed that he was using it when he purchased the gun," the juror said.