In order to arrest George Zimmerman for the shooting death of Trayvon Martin, the special prosecutor heading the investigation must show a judge that she has found probable cause.

Sanford police faced public outrage when they announced they found no probable cause to arrest the Neighborhood Watch volunteer.

So what exactly is it?

"It's a 'reasonable person' standard under the law," said John Tanner, former state attorney in the 7th Judicial Circuit, which includes Volusia County.

It is evidence that would convince a reasonable person that a suspect committed a crime.

For example: It's a rock of crack cocaine found in a man's pocket. It's a department-store security video showing a woman slipping a necklace into her handbag. It's a blood test showing a driver's blood-alcohol level above the legal limit.

Bob Dekle, who prosecuted serial killer Ted Bundy and is now a professor at the Levin College of Law at the University of Florida, characterized probable cause as "just above suspicion."

In the case of Trayvon's shooting death, it would be any piece of evidence that would convince a judge that Zimmerman probably committed a crime when he shot the unarmed teen in a gated Sanford community in late February.

That could be a witness, a piece of physical evidence or something else.

Sanford police were looking for evidence of manslaughter. Special Prosecutor Angela Corey will not say what charge or charges her team is reviewing.

Police can and do arrest suspects without probable cause, but judges must then order their release from jail. Senior Judge O.H. Eaton Jr. said that happened at least once or twice every weekend he was on jail duty reviewing Seminole County arrests for the previous 24 hours.

When that happens, police can rearrest the suspect, and prosecutors are free to file charges. But they must bring the case to trial within 175 days of arrest, according to Florida rules of criminal procedure.

That ticking clock is often an incentive for prosecutors to hold off on an arrest, they said. It gives them more time to collect evidence — for example, to get ballistics tests done if a gun is involved or to have fingerprints analyzed.

Prosecutors almost always demand more evidence than cops.

Though an officer needs only probable cause to make an arrest, prosecutors typically want enough evidence for a conviction — enough to convince a jury beyond every reasonable doubt that the suspect is guilty.

It is a natural point of friction between the two branches of law enforcement, said Ric Ridgway, chief assistant state attorney in the 5th Judicial Circuit, which includes Lake County.

"It's probably the single most frequent source of … disagreement between law enforcement and prosecutors, between victim's families and prosecutors," he said. "They look at it and go, 'We know he did it.' As a prosecutor, I say, 'Can you prove it?' "

Evidence standards

Because of those different evidence standards, prosecutors typically kick loose at least a quarter of the cases in which police make arrests, he said.

That happened in the case of four co-defendants in the Jessica Lunsford murder, Ridgway said. The 9-year-old Homosassa girl was abducted, raped and buried alive in 2005. Prosecutors convicted suspect John Couey, the kidnapper and killer, but Citrus County deputies and prosecutors disagreed about what to do with four of his roommates.