With gun store employee Gordon Cleveland on the witness stand, members of the jury saw the Colt Cobra .38 Special that Hunter Biden purchased from Cleveland on Oct. 12, 2018, at StarQuest Shooters in Wilmington, Delaware.

Prosecutor Derek Hines said the encased gun was rendered safe by marshals and asked the judge to allow him to approach with the weapon.

He first brought the gun to the witness stand where Cleveland read aloud the serial number, and then took it to the jury box, where jurors saw the gun that prosecutors argue Hunter Biden obtained illegally by lying on a federal form.

Prosecutors then moved on to question Cleveland about Form 4473, the form that Hunter Biden filled out when he purchased the gun.

Defense attorney Abbe Lowell, cross-examining Hunter Biden's former girlfriend Zoe Kestan, sought to renew his argument that Hunter Biden was not actively using drugs in October 2018 by emphasizing that Kestan wasn't in contact with him at that time and would not know.

Kestan testified that she wasn't in touch with Hunter Biden from Sept. 23, when they were in Malibu together, until November, when he summoned her to Massachusetts while he was trying out a new ketamine therapy program.

As such, Kestan testified she had "no idea" that he had gone back to Delaware, or what he was doing at that time.

"So you didn't see him between those two dates?" Lowell asked.

"No," she said.

Lowell also sought to cast Kestan as an enabler of Hunter's addiction.

"You helped him get drugs?" Lowell asked.

"Yes," Kestan responded.

Prosecutors pushed back on that argument by highlighting their significant age gap: Kestan was 24 at the time of the relationship, while Hunter Biden was 48.

"Twice my age," she testified.

Kestan subsequently concluded her testimony and stepped down from the witness stand. Prosecutors then prepared to called to the stand Gordon Cleveland, the gun shop employee who sold Hunter Biden the firearm in 2018.

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.