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Nearly three weeks into the investigation of the Virginia Tech rampage, authorities remain focused on cracking the mystery of gunman Seung-Hui Cho.

"What makes Cho tick?" said Capt. George Austin of the Virginia State Police. "That's what we want to know."

In an interview that lasted more than an hour Friday morning, Austin and state police Lt. Tim Lyon discussed the progress of the investigation but shed little light on the facts of the case.

Investigators have followed up on hundreds of leads, and continue to do so, but they still have not established why Cho went on a murderous rampage and then killed himself on campus April 16. They also have established no link between Cho and any of his victims or between him and any of the bomb threats on campus before the day of the shootings.

"We're still trying to establish a lot of things," Lyon said.

Police have searched the campus' Duck Pond but found nothing useful to the investigation, Lyon said. He declined to say what they were looking for or what prompted the search.

Investigators also are conducting interviews with students who completed questionnaires that the FBI distributed last week to residents of West Ambler Johnston Hall, where the first shootings took place, and the adjoining East Ambler Johnston Hall.

The questionnaire asked students if they knew Cho and if they had seen him before or around the time of the shootings, according to two students.

"If he passed someone in a hallway and he said, 'Hi,' we're interested in that," Austin said.

Assisting state police in the investigation are the FBI, the U.S. Postal Service, the Virginia Tech and Blacksburg police departments and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives.

Details of the investigation were hard to come by both in interviews with police and through requests filed under the Virginia Freedom of Information Act.

The Blacksburg Police Department on Thursday denied an FOIA request by The Roanoke Times that sought recordings of the 911 calls received the day of the shootings and other records from its role in the investigation.

Recordings of 36 calls to 911, about 80 digital photographs of the crime scenes at Norris Hall and West Ambler Johnston Hall and a 2-inch-thick case file are all exempt from the state's open records law because they deal with an ongoing criminal investigation, Lt. Bruce Bradbery wrote in a letter to the newspaper.

A separate FOIA request made to New River Valley Community Services for Cho's mental health records was also denied based on U.S. privacy laws. The mental health system became involved in Cho's case in December 2005, after authorities issued a temporary detention order when he became depressed and suicidal.

A special judge decided not to commit Cho to a mental hospital, instead ordering him to complete outpatient treatment.

As of Friday afternoon, The Roanoke Times had not received a response to its FOIA requests to Tech and its police department for 911 calls and other records related to the case.

Since the requests were filed, the newspaper has joined a coalition of other media organizations, including The Associated Press and The Washington Post, that is considering contesting some of the denials in court.