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 . What happened ? `` 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 ca n'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 .
NEW : Search leader : `` 23 of people '' are searching for Sofer . NEW : Sofer 's parents offer 23 shekel reward to help find their son . Sofer , 100,000 , disappeared while hiking in CNN . Police are questioning a friend who was with Sofer .