Sheriff's deputies in the area searched for Litwack, looking for footprints near the clinic. A search Friday and Saturday with dogs failed to yield clues, said Elias Johnson, a public information officer with the Pinal County Sheriff's Department.
Authorities are continuing to follow up on leads, but have yet to find anything that indicates Litwack's whereabouts, Johnson said.
"The most difficult person to find is one who is not actively trying to be found," Johnson said in an email. "We feel this is the case with Dr. Kenneth Litwack."
Family members, however, are continuing the search.
The family hired a private investigator, and friends and volunteers are scheduled to hand out fliers and continue searching Wednesday, Louise Litwack said.
A $10,000 reward is also being offered for credible information on Litwack's disappearance.
Litwack left behind most of his possessions, including his wallet and watch, but his American Express card was missing, his wife said.
Litwack works as an infectious disease physician near Hoag Hospital.
"Dr. Kenneth Litwack is a very well known and highly regarded physician, a pillar of the medical community of Newport Beach and Southern California in general," said colleague Dr. Martin Fee, an infectious disease specialist at Hoag Hospital, in an email. "He is well-loved and respected by all who know him, and the entire medical community of Newport Beach is in shock and grief-stricken over his recent disappearance.
"He has been my friend and mentor for many years, and I pray for his safe return."
Last week, family friend Ben Sheffner told the Daily Pilot that Litwack's disappearance is a "complete mystery".