yes, it was a false alarm triggered through the Emergency Broadcast System. From wikipedia:
False alarm of 1971
Despite these safeguards, the system was accidentally activated at 9:33 a.m. Eastern Standard Time on February 20, 1971. Teletype operator W. S. Eberhardt "played the wrong tape" during a test, which sent an activation message authenticated with the codeword "hatefulness" through the entire system, ordering stations to cease regular programming and broadcast the alert of a national emergency. A cancellation message was sent at 9:59 a.m. EST, but it used an incorrect codeword. A cancellation message with the correct word, "impish", was not sent until 10:13 a.m. EST After 40 minutes and six incorrect cancellation messages, the accidental activation was terminated.
This false alarm demonstrated major flaws in the EBS. Many stations had not received the alert but more importantly, the vast majority of those that did either ignored it (because it came at the time of a scheduled test), or did not know what to do in an emergency. Some stations followed the procedures for an activation, but cancelled them prematurely. It is estimated that only 20% of the stations that received the activation followed the procedures completely. While several stations went off the air, the one best remembered was WOWO in Fort Wayne, Indiana, which broadcast the 1971 events as they happened, a recording of which has become available. Another recording of how the error sounded on WCCO-AM in Minneapolis/Saint Paul, Minnesota can be heard on RadioTapes.com.
Numerous investigations were launched, and several changes were made to the EBS. Among them, the on-air alert announcement was streamlined, eliminating one version of the script that warned the audience of an imminent attack against the country (the WOWO broadcast above does not contain the reference to an attack).