State and federal authorities are still searching for a Yale teaching assistant who failed to return home Friday after a Long Island Sound kayaking trip, police said.

Thomas Casey GRD ’05, who friends described as an avid kayaker, was reported missing by his girlfriend late Friday evening, prompting a massive search by local and state officials and the U.S. Coast Guard.

An area resident found Casey’s empty kayak Friday night, and searchers found his paddle and backpack Saturday, the Associated Press reported.

Casey, 35, is a graduate student in political science. He is a teaching assistant for Pauline Jones Luong’s class, “Politics of Identity.”

Sgt. J. Paul Vance, a state police spokesman, said the search between East Haven and Branford is still active.

“I want to make it perfectly clear that we have been out there and we will stay out there until we determine his position or find out that it is no longer beneficial to continue,” Vance said.

Vance said state police are using a sidescan sonar to search the bottom of the ocean floor.

“[The search] is undetermined,” Vance said. “Obviously if we are searching the ocean floor we’re in a recovery mode, rather than a rescue mode.”

Vance said the experts conducting the search will determine when to end the operation.

The Coast Guard, which began searching for Casey Friday evening, suspended its search Saturday, Coast Guard Lieutenant Brian Lefebvre said.

Coast Guard Petty Officer Scott Steinbarger said there was a small craft advisory warning in effect on Friday.

“[The conditions] were rough,” Steinbarger said. “It depends on the ability of the kayaker. The water was pretty choppy.”

Because authorities have yet to make a final determination, Luong said she is still hopeful.

“We have hope that he would survive because he is a very strong person, physically and intellectually,” Luong said.

Political Science Chairman Ian Shapiro and Director of Graduate Studies Frances Rosenbluth sent an e-mail to their colleagues Monday expressing their sadness about the accident.

“Tom was much beloved in the Department, and we will remember him fondly for his endless good cheer and for his efforts to contribute to the social glue during his three and a half years at Yale,” the e-mail said. “Our hearts go out to his family and to his loved ones. We will want to think collectively about an appropriate tribute.”

Tim Pachirat GRD ’06, a TA with Casey for Luong’s course, said in an e-mail that he knew Casey as a colleague and through some classes they took together.

“I think it’s important to continue to speak about Tom in the present tense,” Pachirat said. “The search has not yielded conclusive results and I think it is premature to conclude that he is not still alive and struggling to make it. Tom is a strong person, both physically and mentally, and I believe he has what it takes to overcome whatever calamity he may have encountered on Friday.”

Luong described Casey as well-loved by his students.

“He has a following among the undergraduates because he is a very devoted teacher,” Luong said.