ALLENTOWN, Pa. -

A four-day search in Berks County by law enforcement authorities in May was for evidence in the 1979 disappearance of a then-16-year-old boy, Lehigh County's district attorney confirmed Monday.

Monday was the 35th anniversary of Robert Keck's disappearance, prompting the district attorney, Jim Martin, to remind the public that the investigation remains open and active.

"I urge anyone with information about the case to come forward," Martin said.

Keck, who would now be 51-years-old, was last seen on June 30, 1979, leaving his home and walking on Coopersburg Road.

The 35-year investigation of his disappearance recently led law enforcement authorities to Pike Township in Berks County, Martin said.

"Based upon information we received from cooperating individuals, various law enforcement agencies participated in a four-day search in May 2014 for evidence in connection with the disappearance of Robert Keck," Martin said. "Unfortunately, that search proved fruitless."

Investigators, including Upper Saucon Township's police chief, Robert Coyle, who took the original missing person's report, remain focused on solving the case, Martin said.

"Law enforcement agencies are still diligently following up on any leads, and it is my hope that at some point, we will be able to conclude the investigation and provide Robert Keck's family with answers concerning his disappearance," Martin said.

Anyone with information about the case is urged to call Upper Saucon Township police Detective Tom Nicoletti at 610-282-3064 or Lehigh County Detective Joseph Vazquez at 610-782-3831.