NEW YORK — The judge presiding over Donald Trump’s hush money trial fined him $1,000 on Monday and warned of jail time for future gag order violations while jurors heard detailed testimony for the first time about the financial reimbursements at the center of the case.

The testimony from Jeffrey McConney, the former Trump Organization controller, provided a mechanical but vital recitation of how the company reimbursed payments meant to suppress embarrassing stories from surfacing during the 2016 presidential campaign and then logged them as legal expenses in a manner that Manhattan prosecutors said broke the law.

McConney’s appearance on the witness stand came as the landmark criminal trial, the first involving a former American president, entered its third week of testimony. His account lacked the human drama offered Friday by longtime Trump aide Hope Hicks, but it nonetheless yielded an important building block for prosecutors trying to pull back the curtain on the transactions designed to protect Trump’s presidential bid during a pivotal stretch of the race.

McConney, who told jurors how reimbursement checks were drawn from Trump’s personal accounts, also provided testimony that could help the defense. He acknowledged, for instance, that Trump never asked him to log the reimbursements as legal expenses or discussed the matter with him at all.

His testimony followed a stern warning from Judge Juan M. Merchan that additional violations of a gag order barring Trump from inflammatory out-of-court comments about witnesses, jurors and others closely connected to the case could result in jail time.

The $1,000 fine imposed Monday marks the second time since the trial began last month that Trump has been sanctioned for violating the gag order. He was fined $9,000 last week, $1,000 for each of nine violations.

“It appears that the $1,000 fines are not serving as a deterrent. Therefore going forward, this court will have to consider a jail sanction,” Merchan said before jurors were brought into the courtroom. Trump’s statements, the judge added, “threaten to interfere with the fair administration of justice and constitute a direct attack on the rule of law. I cannot allow that to continue.”

Trump sat forward in his seat, glowering at the judge as he handed down the ruling. When the judge finished speaking, Trump shook his head twice and crossed his arms.

Yet even as Merchan warned of jail time in his most pointed and direct admonition, he also made clear his reservations about a step that he described as a “last resort.”

“The last thing I want to do is put you in jail,” Merchan said. “You are the former president of the United States and possibly the next president as well. There are many reasons why incarceration is truly a last resort for me. To take that step would be disruptive to these proceedings.”