As a policeman, David Osborne sometimes got frustrated when he saw the criminals he arrested go free.

Still, during his five years as a young officer in New Orleans, he realized that arresting people was often easier than prosecuting them.

Osborne sometimes learned that the hard way after he got a job at the Charleston Police Department more than 11 years ago and became a homicide detective.

He caught his first murder case in October 2006, and before the year's end, his work led to three arrests. But witnesses retracted their stories, and his case fell apart. The men were let go.

One of them, Rafael Horlbeck, would turn up again, and Osborne eventually helped send Horlbeck to prison for life.

Another, Solomon Chisolm, became a suspect in several more slayings before becoming a victim of one.

And it was fitting, Osborne said, that he capped his time as the sergeant leading the homicide unit with testimony that helped put Chisolm's killer behind bars. The trial included some of the same hurdles, such as a shortage of witnesses, that stymied some of his past cases.

In his latest job that he started last week, as an assistant solicitor, Osborne aims to help the officers he once worked alongside to carry their cases to fruition.

"The puzzle portion of ... figuring out what happened, I'm going to miss that," Osborne, 40, said. "But just because you know who did it doesn't mean it's ready for the courtroom. That's the new challenge for me."

Osborne, who will prosecute Charleston crimes, is one of two city police detectives to join the Ninth Circuit Solicitor's Office in January. Lauren Mulkey, who had specialized in robbery investigations, will work as assistant solicitor on cases from Mount Pleasant and unincorporated Charleston County.

Their arrival was a coincidence, Solicitor Scarlett Wilson said, not part of a recruitment of officers with law degrees. It was their attention to detail as investigators, Wilson said, that previewed their prospects as prosecutors.

"With David, it's been a five-year audition," she said. "He's always prepared for anything. For prosecution, that's the name of the game."

Getting results

At the New Orleans Police Department, Osborne joined the "Jump Out Boys," a special-operations team that targeted violent offenders and drug dealers. He spent much of his last two years in the Big Easy as a member of an FBI task force investigating public corruption.

Looking for better schools for his son and daughter, now 12 and 14, he moved in 2002 to his hometown, Charleston.

He worked his way through the patrol and robbery divisions and joined the Violent Crimes Unit in 2006.

In that role, Osborne came across cases that were short on evidence. Witnesses would say one thing during a police interrogation, he said, but refuse to testify at trial. He found himself explaining to victims' families that the charges for which he arrested suspects might not stick.

His first homicide case came in October 2006, when 22-year-old David Hamilton was fatally shot outside a downtown apartment complex. He solved the case, but the witnesses backed out, and the three suspects never were prosecuted. That outcome would help define his career.

"When we made an arrest, it wasn't always ready for the courtroom," he said. "We hoped we could gather more evidence. Sometimes it works, sometimes it doesn't."

Horlbeck, a member of the trio suspected in Hamilton's killing, was known at the time as one of the most violent players in downtown Charleston's drug game. His alleged accomplice, Chisolm, developed a similar reputation over the coming years.

When Horlbeck struck three years later, the resulting murder probe became one of the crowning moments in Osborne's career.

Osborne pressed for statements from two accomplices in the slaying of 15-year-old Jermel Brown in July 2009. The investigation revealed that Brown's brother had lost $200 worth of marijuana that Horlbeck had given him to sell. Horlbeck kidnapped the boy, then shot him under a downtown overpass when the teen tried to get away.

A jury found Horlbeck guilty, and he was sentenced to life in prison.

Another case that involved the death of a "truly innocent victim," Osborne said, arose after he took over as the head of the homicide unit in 2011.

After 17-year-old Marley Lion was slain in June 2012, Osborne helped direct about 50 investigators and police informants who hit West Ashley's streets in search of information. They posted fliers and checked businesses for surveillance video.

The six-month stint he had spent as the sergeant for the Field Intelligence Unit gave him insight into the covert efforts through which informants bought the murder weapon on the street.

Lion's shooter, Ryan Deleston, was sent to prison for life last fall.

"Those were the two most senseless murders I've ever been a part of," Osborne said. "But not only did the investigation come together nicely, we were also pleased with the results."

Changing path

The homicide unit Osborne served on caught flack in June 2009 when the family of 28-year-old Kate Waring said police didn't take her disappearance seriously.

Instead of the police, investigators for the family's attorney, Andy Savage, ended up finding Waring's body on Wadmalaw Island.

That led to stiff cross-examination of Osborne when he testified against Ethan Mack, charged with drowning Waring in a bathtub.

To Mack's attorney, David Aylor of Charleston, Osborne had something not common in all police officers - an understanding of how things work in a courtroom.

"We went back and forth there for a few days," Aylor said of Osborne's testimony during Mack's trial in October 2010. "But he was always up front. ... He wouldn't just read black and white words on a report."

When Mack was convicted, Osborne had already been a part-time student at the Charleston School of Law for more than a year. He took classes at night and investigated homicides by day.

For a man who, as a boy, idolized crime fighters and comic-book superheroes, Osborne said he didn't consider becoming a lawyer until he saw prosecutors work on the cases he investigated. Making the decision to leave the force, though, wasn't easy.

The slayings he solved throughout his four years in law school helped affirm his new path, he said. In the Deleston trial, he said, prosecutors elicited statements from witnesses to better portray what happened on the night of the crime.

To Osborne, the prosecutors were "cops who just dressed like attorneys."

"They were so tenacious and tough on violent offenders," he said. "It opened my eyes to how things were done on that side of a case. It made me want to be a part of their team."

Benefit for all

Osborne's new education was no secret at the Police Department.

He received $3,000 a year in tuition aid from the city. When his workload in violent crimes didn't call for his presence on the streets or in an interview room, he spent his Friday nights and weekends doing schoolwork.

During his schooling, Osborne and Wilson often talked about a career change. Wilson saw how Osborne could fill voids in cases bound for trial.

"He sees what's missing, and he knows where to get it," Wilson said. "He knows what it takes to turn an arrest into a conviction."

His testimony against Lion's killer in October reaffirmed Wilson's decision to bring him aboard, she said. And Wilson's closing argument, which built to a crescendo that labeled Deleston as a cold and callous murderer, reaffirmed Osborne's career switch, he said.

He just needed to take the bar exam. Police Chief Greg Mullen gave him time off to study for the test, which he passed in November.

But when he told Mullen that he was leaving the force, a decision Osborne didn't make lightly, the chief jabbed at Wilson for luring another investigator from his agency.

Mulkey, the widow of Charleston firefighter Louis Mulkey, who died in the 2007 Sofa Super Store fire, was one of the detectives who scoured the Ardmore area of West Ashley for surveillance footage of Lion's killers. She found one key video of an accomplice scouting out the scene, and she testified about it at Deleston's trial.

Wilson said Mulkey's detailed reports made her cases easier to prosecute. Wilson knew that Mulkey, who graduated from the Charleston School of Law and became a bar member in 2008, could have a long career with her office.

And while Mullen told the detectives he was happy for them, the chief said he wasn't happy about their decisions.

Osborne had mentored younger investigators and officers during his time on the force, Mullen said. He was Officer of the Year in 2012 and his unit also won accolades that year, when it cracked the Lion case.

Sgt. Craig Kosarko, who worked some high-profile cases alongside Osborne, will take over as the unit's leader.

"(Osborne and Mulkey) leaving is loss for us, but it will benefit us," Mullen said. "They'll make an impact in the community by prosecuting cases, just like they did in the Police Department."

Osborne's time testifying against killers, though, might not be over.

Of the 12 homicides he helped investigate in 2012, his unit made arrests in 11. He could be called to the witness stand if any more go to trial.

As a detective, his role as a witness came full circle when he testified against Tyrel Collins, the man who fatally shot Chisolm in October 2011.

Osborne had arrested Chisolm in the first murder he handled in 2006, only to see him go free. Despite that adversarial relationship, Osborne said he used as much care in investigating Chisolm's death as he did Lion's.

He called Chisolm's mother on the day Collins was arrested. Rose Chisolm, 59, said Osborne looked beyond her son's checkered past.

He still keeps in touch with the woman as Collins serves a life term behind bars.

"From the night my son got killed, he would call me," she said. "He was always determined to do something. And when he's determined, he does it."