MIAMI, Fla. — It has been months since a face-chewing attack in Miami that left a homeless man hospitalized with portions of his face missing, but many questions still remain.

Rudy Eugene, who was shot and killed by police, is accused of brutally attacking Ronald Poppo during Memorial Day Weekend. But according to new reports, this wasn’t the first time the two met.

According to the Miami Herald, Eugene’s friend Christian says the two had met Poppo before while volunteering with Miami’s homeless community.

“[Ronald Poppo] seemed like a nice and kind man,” Christian exclusively told the Herald. “I remember when we gave him food.’’

Eugene was not on “bath salts” or synthetic marijuana when he chewed the face off Poppo, according to toxicology reports.

But many scientists and skeptical observers don’t believe the reports.

Broward Sheriff Al Lamberti is just one of the doubting Thomases who think the so-called “Causeway Cannibal” was on something not caught by either of the two labs that ran the toxicology tests.

“We are not testing for everything that may be out there,” said Dr. Barry Logan, one of the nation’s leading toxicologists.

That’s because they can’t.

Clandestine labs are using more than 100 chemical compounds to make synthetic marijuana, but even the most sophisticated lab can only test for 17, said Logan, director of Forensic and Toxicological Services at NMS Labs in Pennsylvania, the same lab hired by Miami-Dade County to help test Eugene for bath salts and synthetic marijuana.

Bath salts, also known as synthetic amphetamines, are also hard to track for the same reason.

There are hundreds of bath salt compounds out there, but toxicologists can only test for 40, Logan said.

“This is always a moving target,” Logan said. “As soon as a test exists for something, there are new compounds waiting in the wings. We are always a step behind.”

Even Logan was surprised when Eugene’s drug scan found only traces of marijuana.

“His behavior was consistent with someone who was delusional and hallucinating, which would be consistent with bath salts,” Logan said.

The report released last week by the Miami-Dade County Medical Examiner contained this disclaimer: “Within the limits of current technology by both laboratories, marijuana is the only drug identified in the body of Mr. Rudy Eugene.”

Experts say there is no question Eugene’s behavior was drug-induced – and not by marijuana. But it’s hard to prove because even the most sophisticated labs cannot test for every compound.

“We are not incompetent,” said Dr. Bruce Goldberger, professor and director of toxicology at the University of Florida. “We have the tools, we have the sophistication and know-how. But the field is evolving so rapidly it is hard for us to keep track. It’s almost as if it is a race we can never win.”

Goldberger thinks Eugene was on a drug far stronger than marijuana the day of the attack.

“To say marijuana could have induced this behavior is simply outrageous,” Goldberger said. “No matter how sick mentally or physically a person is, they don’t go around eating people’s faces, or barking at police, or eating a dog, like what happened recently in Texas.”

A Waco man who tried to eat a dog on June 14 told police he was high on synthetic marijuana at the time.

Users say they are drawn to fake weed because it gets them high and doesn’t show up in most drug tests.

One chronic pot user said she’d been smoking synthetic marijuana for two years, then was rushed to the emergency room when she tried to stop cold turkey.

The hospital tests found only traces of marijuana – just like with Rudy Eugene.

Ann Howard, a spokeswoman for the Florida Department of Corrections, said probation officers have the option of testing for designer drugs.

But the tests are expensive – anywhere from $200 to $300 for synthetic marijuana and up to $500 for bath salts, Goldberger said.

Probation officers may not test everyone, but they will target high-risk candidates, said Jim Hall, director of the Center for the Study and Prevention of Substance Abuse at Nova Southeastern University in Davie.

It’s for their own good, he said.

“These are the guinea pig drugs of 2012,” Hall said. “The people using these drugs are risking their lives, their minds and their kidneys. Some of these people have had to be put on dialysis for the rest of their lives.”

Oakland Park resident Jimmy Hewett says his probation officer had him tested for synthetic marijuana after he was quoted in the Sun Sentinel saying he smoked the stuff.

A judge issued a warrant for his arrest after he admitted using the designer drug.

Because it is openly sold at gas stations and convenience stores, Hewett says he didn’t think he was doing anything wrong.

But on July 17, he will have to answer to the court. The charge: Violating probation.