LA CROSSE, Wis. -

Testimony in day four of the Cain Moss trial called into question a key part of the prosecution's case.

The defense called in an expert who is questioning the cause of death of Anthony Ducharme whose body was found in a Northside La Crosse park in 2011.

The autopsy lists the drug Fentanyl as the cause of death.

The prosecution says Ducharme got the Fentanyl from Moss and that makes Moss responsible for his death.

But the defense expert testified there were several chemicals in Ducharme's system and doesn't think Fentanyl is to blame. "My initial report, which this is, states that there are questions about certain things in the case, and that my opinion was that the Fentanyl as listed in the autopsy report is in the therapeutic range and in my opinion would not have caused his death," said Forensic Consultant James Oehldrich.

The prosecution called a rebuttal witness who says Fentanyl was to blame.

The trial will continue Friday with closing statements before being handed over to the jury.