Both Vice President Joe Biden and Defense Secretary Leon Panetta plan to visit the Flight 93 National Memorial in Somerset County, Pa. this week, according to the National Park Service.
Biden was scheduled to speak Tuesday, and the agency advised visitors to get there at least two hours before the 9:30 a.m. start because of security.
Panetta was to take a private tour of the memorial Monday.
More and more people are visiting the memorial in rural western Pennsylvania, authorities say, and new construction is scheduled to begin next year.
At the current pace, more than 1,000 bus groups will visit the park this year, almost double the number in 2011, said King Laughlin, vice president of the Flight 93 National Memorial Campaign. More than 200,000 people are expected to visit this year.
``It really seems to resonate with school groups and bus groups,'' Laughlin said Friday.
Park officials hope to break ground on a visitor center next spring, Laughlin said, as well as tree plantings and new walkways, all scheduled to be completed by 2014. A third phase, which could cost $5 million or more, will include a learning center and a tower at the entrance with 40 wind chimes. Price estimates for the whole project ranged from $67 million to $76 million.
Many improvements have been made since the park opened last year, including reforestation and other landscaping, said Patrick White, president of Families of Flight 93.
``It is coming forward like a painting. Each stroke of the brush brings it more to life,'' he said.
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., after passengers fought back.