ST. PETERSBURG — At 13, Harold Hempstead made a deal with the police.

He would tell a detective about who was selling drugs and committing crime at his school and in his neighborhood. In return, the detective would pay him.

At 16, he was introducing detectives to suspected drug dealers. One included a juvenile detention center employee he had met while in custody. Detectives made plans to sell the employee cocaine. When he got spooked, they asked Hempstead to step in and assure the employee that cops weren't involved.

These accounts by Hempstead are part of a book coming out Sunday, exploring his work as a paid informant in the early 1990s and titled Used and Abused by the St. Petersburg Police Department: My Life as a Juvenile Confidential Informant.

The book aims to help pull back the curtain on the use of civilians in illegal activity — often drug buys — to help police catch criminals.

Hempstead, now serving a lengthy prison term for burglary, was young for the job of informant when he started.

The work can be dangerous, especially for children who don't have the experience or maturity to assess risks and act accordingly, experts say. What's more, a shroud of secrecy around the use of confidential informants makes it hard to hold police accountable when they turn to kids for help.

"The use of juvenile informants is highly unregulated and we know very little about it," said Alexandra Natapoff, a University of California Irvine College of Law professor and a national expert on confidential informants. "The only way that we learn is because something terrible happens."

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St. Petersburg police detectives haven't used a juvenile informant in at least 20 years, a spokeswoman said. But the practice still is permitted under agency policy and Florida law, despite a sweeping reform bill passed a decade ago in response to the murder of 23-year-old Rachel Hoffman during an undercover drug buy.

Several Tampa Bay law enforcement agencies still allow the practice, although officials say it's strongly discouraged, rarely used and requires permission from parents.

Still, Hempstead, now 42, is airing his experiences as a confidential informant in hopes they might encourage young informants to speak out and even lead to a ban on using them.

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In an interview from prison, Hempstead said, "No child should have to live in a constant state of fear of where he can and cannot go because of something that police have gotten him into."

At one time, Hempstead was a normal kid, growing up in St. Petersburg in a tight-knit family, son of a house painter and one of three siblings, according to a book manuscript he provided to the Tampa Bay Times.

When Hempstead was 7, his dad died from health complications. Later, his mom had a breakdown.

Hempstead was attending Sixteenth Street Middle School, now called John Hopkins, when he ran into trouble with the law over thefts from a department store. He met Detective Michael Brown, who asked if Hempstead would still commit crimes if he had money. Hempstead said no. Brown told Hempstead to call him after juvenile detention.

Hempstead's mom encouraged him to follow up, worried he could get in more trouble if he didn't. Mother and son signed consent forms to make it official.

In an interview, Brown said he remembers Hempstead but doesn't recall he was as young as 13 when he started with police.

"I would highly question that," said Brown, who resigned from the department in 2001.

He added that he didn't remember Hempstead as a reliable informant.

"He talked a lot, he had a lot of information, but I don't remember anything ever panning out," Brown said.

The Times could not verify Hempstead's claim that he started at 13, but did obtain police and court records confirming he was a confidential informant as far back as 1992. He was 16 at the time.

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Hempstead's account of his encounter with Brown deviates from a more common path for confidential informants.

Florida statutes define a confidential informant as someone who, in part, "seeks to avoid arrest or prosecution for a crime, or mitigate punishment for a crime in which a sentence will be or has been imposed."

Take Rachel Hoffman, a graduate of Clearwater Countryside High School and Florida State University whose death spurred the restrictions in "Rachel's Law."

Tallahassee police officers found marijuana and ecstasy in her apartment. She agreed to become an informant to avoid more serious charges. Police gave her $13,000 to buy ecstasy, cocaine and a gun from suspected drug dealers. The operation went wrong as officers lost contact with her. Her body was found two days later.

The law named in her honor was among the first of its kind with protections for confidential informants. It includes rules for recruiting and maintaining informants and requires police to take into consideration a potential informant's age and emotional stability.

The only part specific to juveniles is a requirement for supervisory approval to use one.

Lance Block, a Tallahassee lawyer who helped lobby Rachel's Law, drafted another bill in 2015 that would have restricted the use of informants younger than 18 in law enforcement-controlled buys or sales. It also called for agencies to submit reports including the demographics and number of informants and how many were hurt or killed.

"We need to have data on this subject matter," Block said, "because we need to know: What are the costs, and what are the benefits, and who is being used?"

The bill died in committee.

Natapoff, the University of California professor, suggested another level of oversight: Requiring court permission when a police official seeks to use a juvenile informant.

"Law enforcement tells us they should have full unfettered discretion and we should leave them alone to do the job," Natapoff said. "We would almost never accept that answer in any other arena of government."

States such as California and North Dakota, moved by their own tragedies, have adopted restrictions on use of informants 12 and under and 15 and under, respectively.

But a blanket age ban isn't practical in a world where every case is different, said St. Petersburg Assistant Chief Antonio Gilliam, a former detective and supervisor in vice and narcotics.

Gilliam pointed to investigations into threats against schools — a pressing issue in today's world, as an example of the need for juvenile informants. Under St. Petersburg policy, detectives who want to use a juvenile informant must get layers of approval up to assistant chief.

"I would never want to hinder an individual, even if it's a teenager, from providing law enforcement with that type of information," Gilliam said, adding later, "Law enforcement can't solve the majority of crimes without public assistance."

More often than not in St. Pete cases, Gilliam said, defendants approach the police to work as informants, not the other way around, as in Hempstead's account. Informants typically are placed in a criminal environment they're already familiar with and have the mental fortitude to handle.

Hempstead emphasized that he doesn't want to bar kids from talking to the police. But he doesn't think they should be used in undercover operations.

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Hempstead has served about 20 years of a 165-year prison sentence he received for 34 burglary charges, but incarceration hasn't stopped him from speaking out.

In 2014, from the Dade Correctional Institution in Homestead, he tipped off the Miami Herald about an inmate named Darren Rainey who was forced by prison guards into a scalding shower as punishment.

A Herald investigation into Rainey's death spurred reforms of the Florida prison system and a U.S. Department of Justice inquiry. For his safety, Hempstead was transferred to a prison in Tennessee where he is currently incarcerated.

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The book on confidential informants, available for purchase starting Sunday on crusaderbooks.com, is Hempstead's second written from behind bars with the help of associates on the outside. The Herald once labeled him the "Caged Crusader."

Hempstead's informant experience started off rocky, he wrote.

Early on, he called Detective Brown at a pay phone in front of a group of friends and neighbors who overheard him informing on drug dealers, according to his book. A few days later, two of the dealers' customers beat up Hempstead and his sister. He suspects one of the people who overheard the call told on him.

"I was really incredibly ignorant to do such a thing," he said, "but I didn't know better."

Police records recount cases in which Hempstead, at 16 and 17, introduced detectives to suspected dealers and helped facilitate transactions. The records reflect payments between $40 and $400 made to Hempstead, identified as informant 882.

At the time, Hempstead said, there was a certain thrill that came with working as an informant. But as an adult, he said, he realizes he didn't know the risks.

"I guess that's something now I look back on for kids. They should not chase the excitement of a life not knowing all the negatives that come with it."

Times senior researcher Caryn Baird contributed to this report. Contact Kathryn Varn at kvarn@tampabay.com or (727) 893-8913. Follow @kathrynvarn.