The search for David Gimelfarb, the missing 28-year-old U.S. doctoral candidate, continued in Costa Rica this week without the help of two U.S. Army Blackhawk helicopters that had arrived last Wednesday.

Both helicopters returned to their base in Honduras on Sunday after having assisted in the search for three days – from Aug. 20 to 22. The helicopters were fitted with infrared sensors, equipment that officials had told the Gimelfarb family would be the best way to find their son.

A news release from the U.S. Embassy in Costa Rica said that the helicopters did everything possible to aid the search of the Costa Rican Red Cross.

“We regret that we could not find Mr. Gimelfarb,” the release noted.

Meanwhile, local search teams are searching Rincón de la Vieja National Park for signs of Gimelfarb, who was last seen entering the park on Aug. 11 (see TT Daily News report).

A total of 24 rescuers traveled by horseback to the north side of the more than 34,000-acre park Tuesday morning but were deterred from searching because of high winds and low visibility.

Red Cross officials said the north side of the mountain has not been searched on foot as thoroughly as the south side.

Friends of the Gimelfarb family held a vigil in the U.S. city of Chicago, Illinois – Gimelfarb's hometown – on Tuesday afternoon to raise awareness about the doctoral students' disappearance.

Participants marched through the city carrying signs and gathered at Daley Plaza, a popular rallying spot for demonstrations in downtown Chicago.