Manhattan's office real estate market saw its occupancy plunge to is lowest level in five years in 2001, but much of the decline came before the Sept.11 terrorist attacks, a new study says.

The report, to be released later this week by brokerage and tenant representative firm Plymouth Partners, showed Manhattan's vacancy rate more than doubled to 10.9 percent by September 19, 2001, from its 5.1 percent rate in 2000. By year end 2001, it had ticked up to 11.6 percent.

"A maelstrom of negative factors" - the weak economy, the Internet blowup, shriveling advertising revenue and the devastating events of Sept. 11 - all took a toll on the once-hot New York market, the report said. "Corporate downsizing and the implosion of high-tech industries" caused vacancies to shoot up in 2001, it said. While many observers noticed the real estate crunch after Sept. 11, the crumbling actually began much earlier.

"The market was really beginning to tank nine months earlier" as companies consolidated operations, laid off staff and began reassessing their real estate needs, said Mark Thompson, managing director of Plymouth.