Panetta will take a private tour of the memorial Monday, a day before the Sept. 11 anniversary. He will then speak briefly to reporters, the park service said.
Patrick White, the president of a group representing victims' families, Families of Flight 93, said in a statement that they are deeply grateful for Panetta's visit and that it underscores the need to complete the memorial for the 40 passengers and crew members.
"The heroic actions of the Flight 93 passengers and crew members strengthened America," White said. "Secretary Panetta's visit underscores their remarkable, patriotic contribution to this nation, as well as the need to complete the memorial."
The first phase of the project cost $30 million and was finished last year. That includes a memorial plaza and a wall of names.
Mike Litterst, a park service spokesman, said $32 million in public and private money is on hand to complete the second phase, including approaches, walkways and a visitors' center. Work is to begin next year and finish in 2014, he said. A third phase, which would cost $5 million, will include a 93-foot tower at the entrance with 40 wind chimes.
United Flight 93 was traveling from Newark, N.J., to San Francisco when it was hijacked by four terrorists. The 9/11 Commission said the terrorists likely wanted to crash the plane into the White House or the U.S. Capitol, but the jet went down in a field near Shanksville, Pa., as passengers fought back.