Carole Sund, 43, her 15-year-old daughter, Julie, and Silvina Pelosso, a 16-year-old family friend from Argentina, were last seen Feb. 15 in rural El Portal, leaving a restaurant near the lodge where they were staying.
The woman and the two teens were reported missing Feb. 17, a day after they missed a rendezvous with other family members at San Francisco International Airport. A black leather billfold containing Mrs. Sund's credit cards and various IDs was found Feb. 19 on a city street in Modesto, about a two-hour drive west of El Portal.
Though investigators "don't have...concrete evidence that a crime occurred, we are all operating under the assumption that a crime did occur," agent James M. Maddock said Sunday.
"It would be accurate to say we suspect foul play... I hope that gives you a sense of where we're headed," he said. "I told the family we won't stop until we find out what happened, and I'm confident that we will."
More than 50 FBI agents have been working on the case. The women's relatives had converged on the area to help in the search.
Now, they "understand that the probability of their loved ones being alive after this length of time is very small," Maddock said. "But we always hold out that hope that a miracle could occur."
Mrs. Sund's father, Francis Carrington, told ABC-TV's Good Morning America Monday that the FBI conducted routine lie detector tests on at least three family members without finding anything amiss.
"I feel they've cleared everyone," he said. "I can see no possibility there whatsoever." He said he thought the FBI would solve the case.
CBS News Correspondent Vince Gonzales reports that Carole Sund's husband, Jens Sund, and several members of her prominent Northern California real estate family, the Carringtons, are offering a $250,000 reward for information leading to the safe return of the women.
"It's been a constant roller-coaster ride of emotions, fears, and hopes," Jens Sund said on Sunday.
Sund said his wife would never change her plans on a whim.
"She organizes my life, people's lives around hers, the children. It's just very unusual that she wouldn't stick to her planned itinerary," he said.
Raquel Pelosso, a longtime family friend of the Sunds, spent a 19-hour journey from Argentina to the U.S. worrying about her daughter, reports CBS affiliate KOVR-TV in Sacramento.
"At the beginning, we thought it might have been an accident," she said earlier this week. "Something else may have happened. I don't know."