“We didn’t know they could do this much,” said a network engineer in Tehran. “Now we know they have powerful things that allow them to do very complex tracking on the network.”

Deep packet inspection involves inserting equipment into a flow of online data, from emails and Internet phone calls to images and messages on social-networking sites such as Facebook and Twitter. Every digitized packet of online data is deconstructed, examined for keywords and reconstructed within milliseconds. In Iran’s case, this is done for the entire country at a single choke point, according to networking engineers familiar with the country’s system. It couldn’t be determined whether the equipment from Nokia Siemens Networks is used specifically for deep packet inspection.

All eyes have been on the Internet amid the crisis in Iran, and government attempts to crack down on information. The infiltration of Iranian online traffic could explain why the government has allowed the Internet to continue to function — and also why it has been running at such slow speeds in the days since the results of the presidential vote spurred unrest.