According to a more precise description contained in a classified NSA inspector general’s report, also obtained by The Post, PRISM allows “collection managers [to send] content tasking instructions directly to equipment installed at company-controlled locations,” rather than directly to company servers. The companies cannot see the queries that are sent from the NSA to the systems installed on their premises, according to sources familiar with the PRISM process.

Crucial aspects about the mechanisms of data transfer remain publicly unknown. Several industry officials told The Post that the system pushes requested data from company servers to classified computers at FBI facilities at Quantico. The information is then shared with the NSA or other authorized intelligence agencies.

According to slides describing the mechanics of the system, PRISM works as follows: NSA employees engage the system by typing queries from their desks. For queries involving stored communications, the queries pass first through the FBI’s electronic communications surveillance unit, which reviews the search terms to ensure there are no U.S. citizens named as targets.

That unit then sends the query to the FBI’s data intercept technology unit, which connects to equipment at the Internet company and passes the results to the NSA.

The system is most often used for e-mails, but it handles chat, video, images, documents and other files as well.

“The server is controlled by the FBI,” an official with one of the companies said. “We do not offer a download feature from our server.”