LOS ANGELES (CBS) — Attorneys for Michael Jackson’s doctor, Conrad Murray, are expected to open their case Friday morning.

The defense, which is expected to call around 15 witnesses, could wrap up its case by the middle of next week.

“Don’t expect the defense to try to confront this mountain of evidence that’s been building against Dr. Conrad Murray over the course of the last three weeks — they won’t. What they will do instead is try to narrowly focus their attack on the two minutes when Dr. Conrad Murray left Michael Jackson’s side,” CBS News legal analyst Trent Copeland said.

The prosecution’s star witness, propofol expert Dr. Steven Shafer, has been on the stand this week.

Shafer says that for Jackson to have a lethal amount of propofol in his system, it must have been administered by an IV drip.

Murray has said he thinks Jackson gave himself a deadly injection of the drug.

“The theory will also go on to say that Conrad Murray could not have been responsible for his death because Michael Jackson’s own conduct was a superseding, intervening factor that contributed to his death,” Copeland said.

Shafer testified that it would have been impossible for Jackson to have killed himself that way.