The tests detected only marijuana in the system of the attacker, the medical examiner said Wednesday, ruling out other street drugs that some had speculated 31-year-old Rudy Eugene might have taken.

An expert on toxicology testing said marijuana alone wasn’t likely to cause behavior as strange as Eugene’s.

“The problem today is that there is an almost an infinite number of chemical substances out there that can trigger unusual behavior,” said Dr. Bruce Goldberger, Professor and Director of Toxicology at the University of Florida.

Goldberger said the medical examiner’s office in Miami is known for doing thorough work and he’s confident they and the independent lab covered as much ground as possible. But it’s nearly impossible for toxicology testing to keep pace with new formulations of synthetic drugs.

“There are many of these synthetic drugs that we currently don’t have the methodology to test on, and that is not the fault of the toxicology lab. The challenge today for the toxicology lab is to stay on top of these new chemicals and develop methodologies for them, but it’s very difficult and very expensive.” Goldberger said. “There is no one test or combination of tests that can detect every possible substance out there.”

There has been much speculation about what drugs, if any, would lead to the bizarre behavior authorities said Eugene exhibited before and during the attack that left the other man horribly disfigured. A Miami police union official had suggested that Eugene, who was shot and killed by an officer during the attack, was probably under the influence of bath salts.

The Miami-Dade County Medical Examiner said in a news release that the toxicology detected marijuana, but it didn’t find any other street drugs, alcohol or prescription drugs. Eugene also tested negative for adulterants commonly mixed with street drugs.

The department ruled out the most common components found in so-called bath salts, which mimic the effects of cocaine or methamphetamine and have been associated with bizarre crimes in recent months. An outside forensic toxicology lab, which took a second look at the results, also confirmed the absence of bath salts, synthetic marijuana and LSD.

Messages left with the medical examiner’s office for comment were not immediately returned.

The Drug Enforcement Administration last year temporarily outlawed the possession and sale of three synthetic stimulants sometimes packaged as “bath salts.” Several states have also moved to ban the drugs, often sold on the Internet and in head shops and other retail outlets. The bans don’t affect the kinds of bath salts added to tubs for their fragrance and cosmetic benefits.

An addiction expert said she wouldn’t rule out marijuana causing the agitation.

“It could have been the strain of marijuana that increases the dopamine in the brain, such as sativa,” said Dr. Patricia Junquera, assistant professor at the Department of Psychiatry at the University of Miami Miller School of Medicine.

There are two strains of marijuana called sativa and indica. The sativa increases dopamine and gives you energy while decreasing pain threshold. Indica is a “sleepy high,” she explained.

“People don’t really know what the amount of either is in each little packet of marijuana,” she explained. “And we can’t differentiate between the two in the blood, much less in a dead person.”

She also suggested that if Eugene had a mental disorder, “the marijuana could have increased even further the dopamine levels and aggravated the situation. So that can’t be ruled out.”

It’s not clear what led to the May 26 attack on Ronald Poppo, a 65-year-old homeless man. Eugene’s friends and family have said he was religious, not violent and that he didn’t drink or do drugs harder than marijuana.

“There’s no answer for it, not really,” Eugene’s younger brother, Marckenson Charles, said in an interview. “Anybody who knew him knows this wasn’t the person we knew him to be. Whatever triggered him, there is no answer for this.”

Surveillance video from a nearby building shows Eugene stripping Poppo and pummeling him, before appearing to hunch over and lie on top of him. The police officer who shot Eugene to death said he growled at the officer when he told him to stop.

Charles, Eugene’s brother, said the family does not plan to pursue any legal action against the police for shooting Eugene.

“They used the force they felt was necessary, even if we don’t agree with that,” he said.

He said Eugene has been buried.

Shortly before the attack, a person driving on the MacArthur Causeway told a 911 dispatcher a “completely naked man” was on top of one of the light poles on the causeway and “acting like Tarzan.” Still, police have said little about what may prompted Eugene to attack Poppo.

Poppo has undergone several surgeries and remains hospitalized. His left eye was removed, but doctors said earlier this month they were trying to find a way to restore vision in his right eye. He will need more surgeries before he can explore the options for reconstructing his face, doctors have said. A message left with the hospital was not immediately returned.

Poppo’s family has said it had no contact with him for more than 30 years and thought he was dead.

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Associated Press writer Jennifer Kay contributed to this report.