The first time Derrick Spivey learned about “Bradying” was at his own preliminary hearing, facing a charge of first-degree murder.

The sole witness at the 2009 hearing was a former high school classmate of Spivey’s who, a year earlier, had signed a statement that Spivey fatally shot Marvin Hudson, 21. Now that witness, Rahman “Rock” Isaac, testified that his statement was a fabrication by detectives, one that he’d signed under duress. In fact, he testified, he had ducked for cover and did not see who shot Hudson.

Still, based on the disputed statement, a Municipal Court judge ruled the prosecutor had made out a strong enough case to take it to trial.

Such a ruling was possible because of a 1986 Pennsylvania Supreme Court case, Commonwealth v. Brady. That landmark case placed Pennsylvania among a minority of states that allowed prior inconsistent statements to be presented as substantive evidence in court, even over the witness’ denials. That means a judge or jury can accept that statement as the truth — a significant difference from other states that allow such statements in court only to challenge the credibility of the witness.

In other words, said R. Michael Cassidy, a former prosecutor and a Boston College law professor, “it’s up to the jury to decide whether they’re lying now or lying then.”

Critics say being able to use recanted statements gives police an incentive to get signatures on statements at any cost — even if the witness may later claim coercion. Craig Cooley, a lawyer who handles appeals, said he sees the practice as a major driver of wrongful convictions:

“You bring [a witness] down to homicide, you keep him there for umpteen hours, you make him sign a statement. Then, when he gets on the stand to testify and he says, ‘I didn’t say this,’ under state law you can introduce it as a prior inconsistent statement. And a detective can summarize what he allegedly said.”

In the years since Brady, introducing prior statements as a way to deal with recanting witnesses grew so routine in Philadelphia criminal trials that some detectives and prosecutors began referring to it simply as “Bradying a witness.” They say it’s a valuable tool, since witnesses may recant due to social pressure, threats, or fear of being labeled a snitch.

After Spivey encountered the protocol as a defendant, he experienced it as a witness.

The chain of events began long before his preliminary hearing, on July 4, 2007, when Spivey’s friend Erek Williams was shot and killed in broad daylight outside Spivey’s home. Seven months later and two miles away, Marvin Hudson was gunned down in his car. Passengers Rock Isaac and Karefe Cover were uninjured.

Now, the homicide detectives had a theory: a series of retaliatory killings between two feuding groups of young men. They soon collected statements that backed it up.

Spivey said in an interview that detectives handcuffed him, brought him to the Homicide Unit, and questioned him intensely about Rock’s brother, Raheem Isaac. “They wouldn’t let me go. I was there for two days — no food, no water. I told them the story, but they didn’t care. They were saying that I was lying because I said I didn’t know who did it.”

They returned time and again, sometimes as late as 3 a.m. Spivey’s mother, Melanie Shepherd, said: “The detective kept saying that whoever talks first gets the deal. He came here almost every day.” Finally, Spivey said, Detective Timothy Bass took him back to the Homicide Unit and showed him a picture of Raheem. “He was like, ‘If you don’t know him, you ain’t leaving.’” Spivey said he signed a statement so he could go home. (Bass did not respond to messages.)

Meanwhile, Detective James Pitts questioned Rock about Spivey’s involvement in Hudson’s murder.

At Spivey’s preliminary hearing, Rock testified that he signed the statement only to escape the interview room and that detectives held him for eight hours and slammed his head on the desk so hard it knocked him out.

“He done beat me, like I told you before,” Rock testified under questioning by the prosecutor. “My sister picked me up. I had a hole in my shirt and blood on my T-shirt. He tried to manipulate me. He had called my best friend’s mom and put her on the phone so that I could try to be manipulated to telling something I didn’t see.”

By the time Spivey was called to testify in the murder case against Raheem Isaac, Spivey refused to be Bradyed. He declined to answer any questions. That prevented the statement he had signed from being read into evidence but resulted in a six- to 12-month sentence for contempt.

Both Spivey and Isaac were convicted. Both of their cases were overturned years later in Superior Court. In Spivey’s case, judges found that his lawyer failed to demand a mistrial after improper testimony bolstered the prosecutor’s weak identification evidence. In Isaac’s, they ruled that the trial judge had allowed inadmissible hearsay testimony.

And both pleaded guilty to lesser charges to get out of prison.

Pitts stands by the integrity of both convictions, and denied any assault or coercion. “Which sounds more believable to you? We cased up two people for nothing? For what? … I didn’t know Derrick Spivey until Rahman said, ‘That’s the guy that shot at me.’”

Cassidy, the Boston College professor who specializes in prosecutorial ethics, said that cases involving recanting witnesses demand extra scrutiny from prosecutors to watch out for patterns of apparent coercion.

“It seems to me they need to be particularly on guard in jurisdictions like Pennsylvania that allow the prior statement to come in for its truth,” Cassidy said. “There’s a greater danger of injustice.”