BERLIN (Reuters) - The head of an alternative cancer treatment center in Germany is under investigation after three patients died there in suspicious circumstances, German prosecutors said on Friday.

The non-medical practitioner, identified as Klaus R., is suspected of manslaughter in three cases and negligent injury in the case of two more patients who remain in serious condition, said Axel Stahl, senior prosecutor in the Krefeld prosecutor's office.

Investigators are looking into allegations that the suspect treated patients with 3BP, an experimental drug that has not been clinically tested or approved for use as a cancer treatment. It is, however, used in alternative medicine, and the suspect was authorized to administer the drug on that basis. He has denied any wrongdoing.

"These are people who received treatment shortly before July 19 ... and immediately following this treatment had a dramatic decline in health", Stahl said.

The investigation will mainly focus on whether a causal link existed between the deaths of the patients and their treatment, Stahl said. It will probably be several weeks until prosecutors had the first results, he said.

Investigators would look into whether the chemical composition of the drug was incorrect and whether the dosage could have been wrong, he said.

The alternative treatment center, located in the West German city of Brueggen near the Dutch border, was an outpatient practice. "The patients came mostly from Belgium and the Netherlands," Stahl said.

(Reporting by Reuters TV,; Writing by Michael Nienaber,; Editing by Larry King)