Maddula's phone last pinged off a cell tower in that area -- just about an hour after he left the party, officials said. His family announced a $25,000 reward for information in the case on Wednesday.
"That ping would have been early Saturday morning sometime around 1a.m. or thereafter. Based on that it's believed this is the closest to his last known location," Alan Cubbage, Northwestern University spokesperson, said.
Divers from the U.S. Coast Guard and local fire departments were involved in the water search. Crews using dogs also looked at boats docked in the harbor and the area nearby.
"We have widened the search area because an examination of cell phone activity done by police showed that the closest cell phone tower was the one here at the U.S. Coast Guard station in Wilmette," said Alan Cubbage, vice president of university relations at Northwestern.
How he or his cell phone got to the harbor area in Gillsen Park remains a mystery. It's about two miles north of where the party was- and in the opposite direction of Maddula's dorm on the Evanston campus. The walk would have taken about half an hour.
"The question of whether he left the party alone or whether he was with a group is what university police and Evanston police are trying to nail down right now. We just don't know the answer to that right now," Cubbage said.
Maddula's family, who flew in from New York to search, said the 18-year old is a bookworm-- not an adventure seeker.
"I don't know why he would come at 1 a.m. in the morning. That we don't have any idea," Prasad Maddula, father, said.
They hope the $25,000 reward will generate more information.
"Any information they can give that leads to find Harsha, my son, please, I want my boy back home," Dhannalakshmi Maddula, mother, said.
The 5'8" and approximately 150- pound Maddula is of Indian descent. He has black hair and was last seen wearing black-framed glasses, a gray sweater with black and white stitching, dark gray pants and brown shoes.
The Evanston Police Department and the FBI are also involved in the search. Anyone with information is asked to call Northwestern University Police at (847) 491-3254.