R. Kelly and one of his assistants were in a park in Chicago a few years before his 2019 arrest when the singer asked the assistant, Diana Copeland, why she had not acted to prevent one of his girlfriends from packing her suitcase and leaving his home.

Why, he wanted to know, had Ms. Copeland let the woman “escape?”

That single word in Ms. Copeland’s testimony at Mr. Kelly’s trial on Friday served as a sharp echo following hours of searing testimony from accusers who said Mr. Kelly sexually and physically abused them, imprisoned them and raped them. But along with those accusers, employees like Ms. Copeland — who worked off and on for Mr. Kelly for about 15 years and was the fifth former employee to take the stand against the R&B star — are at the center of the racketeering case against him.

The testimonies of the former employees go to the heart of the government’s contention that Mr. Kelly not only was a predator, but also was the ringleader of a decades-long conspiracy that used his stardom to prey on a conveyor belt of women, men and teenagers, on whom he exacted wilting control.

In her testimony, Ms. Copeland described the bizarre lengths Mr. Kelly went to control the women in his sphere — down to the Uber drivers he allowed them to ride with. If a man pulled up, she said, “I would have to call another one” — until a female driver appeared. Another woman testified that Ms. Copeland had accompanied her to get tested for sexually transmitted diseases and then withheld her test results. (Several women have testified that Mr. Kelly knowingly gave them herpes.)

In a case anchored by the stories of his encounters with six women over several decades, Mr. Kelly, whose real name is Robert Sylvester Kelly, is charged with one count of racketeering and eight violations of an anti-sex-trafficking law known as the Mann Act. The top charge, made famous for nabbing mobsters, enables prosecutors to present evidence of would-be crimes — like Mr. Kelly’s illicit marriage to a 15-year-old Aaliyah in 1994 — that would normally be too old for consideration at trial.

But such a case hinges on former employees testifying to the nucleus of a criminal enterprise swelling well beyond the confines of what Mr. Kelly’s defense lawyers have described as the mere trimmings of a successful music empire.

Jeffrey Lichtman, a lawyer who has represented El Chapo and successfully defended John A. Gotti Jr. of the Gambino family in federal cases in which they were accused of heading criminal enterprises, called the case against the singer “highly unusual.”

“In order to convict a mob boss, they get associates and made men to flip,” Mr. Lichtman said of prosecutors. “But now, this enterprise consists of his employees that did R. Kelly’s bidding: getting him girls and helping him control them.”

The government’s use of testimony from former employees, one of whom received court-ordered indemnification for his time on the stand, is expected to validate the stories of his accusers, whose encounters and relationships with the singer are sometimes fraught with messy, conflicting details.

At times, these employees, some of whom still seemed loyal to the singer and were subpoenaed to testify, have corroborated accusers’ testimonies, acknowledging the numerous, strict rules the women were made to follow, and the abuse they say they endured.

Alesiette Mayweather, one of Mr. Kelly’s former assistants, said Tuesday that at times she feared for the well-being of some of the singer’s live-in girlfriends. In text messages read aloud in court, Ms. Mayweather told his stylist that she was worried about one of Mr. Kelly’s girlfriends, who was briefly returning home to Florida for her high school graduation.

“When she gets to Florida she should run and NEVER come back,” Ms. Mayweather wrote in one text, adding in another: “Something is REALLY strange about the treatment and behavior of the little one. The punishment is REAL!!”

One employee, Tom Arnold, who worked for the singer for about eight years until 2011, told the jury during the second week of the trial that he and others working for the singer were engaged in such a prolific attempt to secure young women for Mr. Kelly that they typed and printed the singer’s phone number onto slips of paper to hand out at malls, concerts and afterparties.

After those women were integrated into his wide circle, Mr. Kelly limited his girlfriends’ interactions with other men to such an extent that Ms. Copeland said the singer once deducted her pay simply because she had scheduled appointments for two of them at a nail salon where a man happened to work.

Mr. Kelly directed the women to turn from other men at shops and in elevators, Ms. Copeland said, noting that she once accidentally left a live-in girlfriend on an elevator because the woman had turned her back to the door when a man boarded and didn’t see Ms. Copeland getting off.

And on Tuesday another employee, Nicholas Williams, who manned the phones at one of Mr. Kelly’s Chicago studios in parts of 2003 and 2004, told the jury that on “numerous occasions” the singer issued a “blackout period”— meaning employees were restricted from venturing upstairs, where Mr. Kelly was entertaining his female guests. Mr. Williams, who was 19 at the time, said the women were “very young.”

Sometimes, Mr. Kelly’s then-wife, Andrea Lee, with whom the singer had three children, would call the studio, Mr. Williams said, adding that he would quickly scrawl the code name Mr. Kelly used for his wife — “Important” — on a piece of paper to bring to the singer.

He ran to Mr. Kelly to deliver the messages, he said: “It was to alert Rob that his wife was coming, so that there would be no young female guests in the hallway.”

The bevy of women living with Mr. Kelly were “not free to roam the house,” Ms. Copeland said, adding that as a female employee she was able to move about, but that freedom came with strings: The singer directed her to knock on the walls as she moved from one room to another “to make sure that I announced my presence.”

During cross-examination, Ms. Copeland said that some of the women in Mr. Kelly’s life came and went as they pleased, and enjoyed what one of Mr. Kelly’s defense lawyers, Deveraux L. Cannick, called his “exotic lifestyle” — including extravagant parties, trips and gifts of exotic pets.

Ms. Copeland also suggested that some of the rules — like knocking on doors before entering — were not the result of Mr. Kelly’s attempts to control people for sex, but were simply his own peccadilloes. For instance, she said that “everybody” — not just his live-in girlfriends — was required to knock on doors before entering. That rule, she said, was created by Mr. Kelly’s then-wife. (Ms. Lee and Mr. Kelly divorced in 2009.)

Ms. Copeland said that despite a slew of arguments that led to her quitting her job on multiple occasions, she ultimately returned to work for the singer several times out of concern for him.

“I felt that he didn’t have trustworthy employees,” Ms. Copeland said.

She said that Mr. Kelly did not have any control over his bank account, “had no idea where his royalties were going,” and did not even know his own Social Security number. She said that during her tenure with Mr. Kelly, she learned that the singer, who crafted his songs in his head, had trouble reading and writing.

Ultimately, despite her concerns for the singer’s well-being, Ms. Copeland quit working for Mr. Kelly for good in April 2018, she said. And sometime in 2019, the same year that Mr. Kelly was arrested, she met with him one last time at his Chicago apartment in Trump Tower, where another woman, who testified under a pseudonym at the start of the trial, “gave me a robe to put on,” Ms. Copeland said, adding: “She asked me if I was wired — if I had some kind of device that would record.”

She said she was shocked by the implication, but stripped down to her tank top and put on the robe.

Mapping a landscape of Mr. Kelly’s workplace culture over decades, the seven employees who have testified so far have described working for the singer in terms that far surpass the ordinary demands of an eccentric artist — sometimes noting that they broke the law to help facilitate Mr. Kelly’s interactions with underage girls.

“It was almost like the Twilight Zone,” Anthony Navarro, a former runner who worked for the singer for some two years, told the jury on the third day of the trial. “You went into the gate, and it was like a different world, just a strange place.”

For Demetrius Smith, who worked as the singer’s tour manager at the start of his career, becoming a part of Mr. Kelly’s world included helping him secure false identification records for Aaliyah, who was then 15 and who Mr. Kelly believed was pregnant with his child, according to court testimony.

Mr. Smith, who was subpoenaed to testify over two days toward the start of the trial, said that while he did not agree with the illegal marriage, he felt pressure to help make it work in order to stay in the singer’s good graces. And, he said, he feared that if he did not contribute to the plan, “I felt like I was going to be pushed out of the loop.”

Over two days of testimony in the third week of the trial, Suzette Mayweather, another of the singer’s personal assistants who described herself on the stand as Mr. Kelly’s longtime friend, said his desire to control the women in his orbit made her suspect that he sometimes used violence to impose his will. (Ms. Mayweather, who is the twin sister of Alesiette Mayweather, said she had never seen Mr. Kelly hit anyone but that she had once heard what sounded like a slap and that another woman had shown her the markings endured from him spanking her.)

Once, after talking with one of Mr. Kelly’s girlfriends about her relationship with the singer, his children and her own aspirations as a writer, Ms. Mayweather said that Mr. Kelly grilled her about the conversations, which she told the jury the other woman had always initiated.

But as Mr. Kelly interrogated her at his studio that day, Ms. Mayweather said, she lied to him, taking responsibility for starting the prohibited conversations, after fearing for the woman’s safety.

“I had never seen Rob that upset, and it wasn’t his tone,” she said. “It was the look in his eyes.”

Troy Closson and Rebecca Davis O’Brien contributed reporting.