The teargas used by interior ministry troops in Cairo's Tahrir Square is supplied by a US company. Demonstrators say cartridges retrieved from the scene are branded with the name and address of Combined Systems Inc (CSI).
The firm is located in Jamestown, Pennsylvania. It specialises in supplying what it calls "crowd control devices" to armies and "homeland security agencies" around the world. It also manufactures lethal military equipment.
Protesters say the CS gas seems more powerful than that used by Egyptian police during the country's last popular uprising in February. "It's stronger, it burns your face, it makes you feel like your whole body is seizing up," one witness said. He added: "It doesn't seem to be combated by Coke or vinegar."
Experts told the Guardian the gas was likely to be standard CS gas, but the effects could be exacerbated by physical exertion.
As well as the effects of the teargas, protesters have suffered grave injuries to their heads and faces from rubber bullets. There are also reports of live ammunition being used. Dozens of people have been taken to makeshift hospitals after inhaling the choking gas fired by the Central Security Forces.