The origin of the broadcast is a ham radio in a military bunker, which can be accessed from a hatch at the base of the southwestern satellite at the Fort Hagen satellite array. Note that this is NOT the same bunker as the one involved in the quest line of Automatron.
“This is a message from the Commonwealth of Massachusetts Emergency Alert System. This morning, Saturday October 23rd, authorities detected that a nuclear attack had commenced against the United States. The following cities affected include, but are not limited to: Washington, DC; New York; Philadelphia; and Boston.
If you are receiving this emergency alert message, immediately seek shelter that can provide protection from radioactive fallout. This has been an emergency action notification.”