The federal agents came in the pre-dawn darkness on Friday to a Michigan home. They had an urgent message for Mike McCulloch, a veteran attorney: His life was in danger.

As FBI agents rolled under his car to search for a GPS tracker, the feds explained that Daniel Schwarz, a doctor who had recently lost about $60,000 in a case McCulloch litigated, was plotting to kill him. His weapon: Fentanyl, they said, the powerful opioid.

"I couldn't believe it," McCulloch told Detroit's WXYZ. "I will tell you that I've been a lawyer for 35 years, and I've never run into people like this."

Schwarz, 56, was arrested on federal drug charges on Friday and in court documents, agents say he admitted to talking about killing McCulloch — and even told the FBI he had a "fantasy" of kidnapping the attorney and torturing him in a scene he compared to the Quentin Tarantino film Reservoir Dogs. But the doctor denied he ever actually intended to kill him.

While doctors and drug dealers nationwide have increasingly faced murder charges in overdose deaths tied to fentanyl, Schwarz's alleged revenge plot could be among the first murder-for-hire cases involving the drug.

Federal agents say Schwarz had ready access to the synthetic opioid, which is largely produced in Mexico and China and is so powerful that just a few bits the size of a grain of salt can be fatal, The Washington Post reported. Schwarz worked in pain clinics around Michigan and Ohio. The FBI was already investigating him earlier this year for conspiracy to possess fentanyl.

But the case took an unexpected turn this month, when a methamphetamine addict who was co-operating with the FBI said Schwarz had approached her about killing McCulloch.

The root of the doctor's rage, the agents said, was a court judgment he'd lost for nearly $60,000. The case, WXYZ reported, was between Schwarz and the condo association in a lakefront community outside Detroit where the doctor owned a unit. McCulloch represented the condo association.

The witness met Schwarz while she was in drug recovery. At a clinic in Findlay, Ohio, the doctor first told her that he wanted to kill the lawyer and plant fentanyl on his body, the FBI said. At first, he also asked the witness to buy a gun with a silencer.

But then, his idea shifted. Instead, the woman could kill McCulloch by giving him a "covert lethal injection" of fentanyl mixed with Xanax, an antidepressant, while "accidentally" bumping into him in public, according to court documents.

"They said she'd been groomed and recruited to carry out a hit on me," McCulloch told the judge in the Michigan condo dispute case on Wednesday. "[The fentanyl] would be stroked on my skin, and I would suffer death."

The doctor also suggested he might frame Robert Meisner, his own attorney in the condo dispute whom he blamed for "a perceived lack of effort," by planting fentanyl in his car.

The witness thought Schwarz was just "venting," according to court documents, until the doctor grew more insistent about the idea. On Aug. 3, the witness went to police in Lima, Ohio, and blew the whistle. When the FBI interviewed her two weeks later, she said Schwarz had offered $2,000 for helping with the crime and had told her to buy a burner phone.

The witness began wearing a wire and recording phone calls in which Schwarz reiterated the plot. He noted that he had only enough fentanyl for one of the victims, the FBI said, and asked the witness to get more.

On Aug. 22, as the FBI listened in, the witness showed up to Schwarz's medical office just outside Toledo with a fake package of fentanyl. Schwarz allegedly talked about how the witness could "sprinkle" the deadly drug on McCulloch's neck and swirl a mixture of heroin and fentanyl into his drink at a coffee shop. But he also noted that he'd found another man with a vendetta against the lawyer who said he'd kill McCulloch for $10,000, the FBI said.

After the witness handed over the fake drugs, the FBI said, federal agents followed him to a gas station and arrested him.

The doctor allegedly confessed to talking about the plot, telling FBI agents that "he fantasized" about killing McCulloch as well as his personal lawyer, and had discussed "kidnapping the attorneys and taking them to a secluded location and burning them, or injecting them with fentanyl."

But he also claimed that the ultimate aim of the plot he'd discussed with the witness was simply to get narcotics into McCulloch's system and to plant drugs in both attorneys' cars, not to kill anyone.

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Schwarz, who is charged with conspiracy and an attempt to possess with the intent to distribute a controlled substance, doesn't have an attorney listed yet in court dockets. He's due back in federal court in Toledo at 10 a.m. Thursday for a detention hearing.

McCulloch said he's nearing retirement and feels fortunate to have escaped the situation unscathed.

"Believe it or not, this was one of the last cases I'm handling," he said in an interview with The Washington Post. "What a way to go out."