Jerusalem (CNN) -- The father of a New Jersey man who went missing while studying in Israel has joined police and hundreds of volunteers searching the forest outside Jerusalem where his son disappeared.
Yeshiva student Aaron Sofer, 23, was reported missing Friday by a friend who was hiking with him in the Jerusalem Forest, Israeli Police spokesman Micky Rosenfeld told CNN.
Sofer's father, Moshe Zvi Sofer, arrived in Israel Monday with his wife from their home in Lakewood, New Jersey, to join the search, which is now in its fifth day.
The couple is offering a 100,000 shekel (about $28,000) reward for whoever finds their son, Sofer's father said in a video posted online.
"It means a lot to us that everyone is doing their share, whatever they could to help us find my son," he told CNN at the search site Wednesday.
Police, volunteers search forest
Avishai Landau, who is organizing the search, said the group of "hundreds and hundreds of people" started combing the forest Saturday.
Landau, who's been volunteering on search and rescue missions for years, called Sofer's disappearance "extremely unusual," and said that the search party has come up empty-handed.
"We have absolutely nothing that could explain what happened to him," he said.
Police have questioned a friend of Sofer's who was "the last man who saw him face-to-face, eye-to-eye," Rosenfeld said.
The friend lost contact with Sofer while walking with him and reported the disappearance to authorities, the police spokesman added.
"We are looking in a number of directions," Rosenfeld said. "It is not clear if the disappearance was personal [or a] kidnapping. It's not clear at all."
The missing man's brother, Joel Sofer, told CNN Tuesday that family members are "just praying hard and doing everything we could do."
He said he had visited the area where his brother went missing.
"I actually was over there today on site," he told CNN's Jake Tapper. "And you know, there's nothing. They have no evidence. They can't find anything."
Binyomin Isseeman, Sofer's rabbi in Lakewood, called the young man a "very normal, a very healthy boy."
"His mind is clear, no problems at all," Isseeman said to CNN affiliate WCBS reported.
U.S. State Department assisting
Republican Rep. Chris Smith of New Jersey said the U.S. Department of State was helping Israeli authorities in the search for Sofer.
"There are concerns, as we all know, about the possibility of abduction," Smith said, according to CNN affiliate News 12 New Jersey.
Sofer's disappearance comes two months after what Israeli authorities have called the revenge killing of Palestinian teen Mohammed Abu Khedair, 16. His body was found in the Jerusalem Forest on July 2, just miles from where Sofer went missing.
Two minors and an adult were indicted in the killing. Prosecutors allege the killing was retaliation for the deaths of three Israeli teens whose bodies were found in the West Bank in June. One of the Israeli youths also had American citizenship.
CNN's Michael Schwartz reported from Jerusalem and Leigh Remizowski reported from New York. CNN's Jethro Mullen also contributed to this report.