Among the largest tenants was Morgan Stanley, which had its retail trading operation headquartered in the south tower of the World Trade Center.
"We have no information beyond what has been reported in the news," the company said in a statement on its Web site. "The safety and well-being of Morgan Stanley employees are paramount at this time."
Real Estate services company CoStar has posted a list of World Trade Center tenants on its Web site.
According to promotional materials, the World Trade Center had a daily population of some 40,000 workers and more than 100,000 visitors and tourists.
In July, the Port Authority of New York and New Jersey leased the World Trade Center, including the two collapsed towers and the retail mall in a 99-year lease worth an estimated $3.2 billion to Silverstein Properties and Westfield America, a publicly traded company based in Los Angeles.
Marriott said on its Web site that the 800-room hotel was evacuated and said it was setting up a hotline for inquiries.