Illinois authorities widened the search Wednesday for a New Hyde Park college student who was last seen leaving an off-campus party early Saturday.

Harsha Maddula, 18, a Northwestern University sophomore, left the party about 12:30 a.m., but closed-circuit TV tapes and electronic key card data show the premed student never returned to his residence hall on the Evanston campus, university officials said.

The FBI and 20 state law enforcement and fire agencies, assisted by Northwestern students, have been searching the campus vicinity. Wednesday morning, they were looking for Maddula two miles north of his dorm.

Searchers focused on Wilmette Harbor, adjacent to Lake Michigan, after learning that a nearby cellphone tower received a signal from Maddula's phone about 1 a.m.

"Authorities have brought in new resources from neighboring suburbs, including more divers . . . and a sonar team from Lake and McHenry counties, to examine the waters in Wilmette Harbor," the university said last night in a statement.

Parts of Lake Michigan were also searched by sonar.

Maddula's parents, Dhanalakshmi and Prasad Maddula, arrived Monday night in Evanston and spoke the next day with Northwestern's police chief and vice president for student affairs, the university said.

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Speaking to reporters Tuesday, the couple urged any students with information about their son to come forward.

"It's been four days," the father said. "My hopes are fading."

In news interviews, the student's aunt, Surekha Maddula, who traveled from New York to help organize searches, said the family fears his disappearance may be related to his recently diagnosed diabetes.

"All of us cousins actually had to beg his parents to let him go back to college, to be honest, because they were concerned that he had this new illness," she told CBS News.

The family Wednesday posted a $25,000 reward on the "Harsha Maddula is missing" Facebook page, created by one of Maddula's cousins, to "anyone who finds Harsha."

The Daily Northwestern, a student paper, reported that the university waited 50 hours before sending out a missing student email alert Monday night.

Alan Cubbage, vice president for university relations, said university policy calls for family to be contacted before the alerts go out, and Maddula's family could not be reached until Monday.

The college paper quoted friends saying Maddula was "coherent . . . and did not appear drunk" when he left the second of two parties last weekend.

During an eight-minute walk to a third party, Maddula became separated from his friends, who noticed him missing when they arrived at the new location. Maddula called another sophomore at 12:10 a.m., saying he was "going to see if he could make it over there," the paper reported.