The head of Hawaii’s emergency management agency has resigned and a state employee who sent out an false alarm of an imminent missile attack has been fired, it was announced on Tuesday, after an inquiry into a mistake which caused statewide panic earlier this month.

According to a federal inquiry into the incident, the employee, a watch officer at the emergency management agency, believed the threat of a missile attack to be real as he had not heard a recorded message announcing it as an exercise. According to state officials, the watch officer had been a cause for concern to his colleagues for more than a decade and had twice before mistaken drills for real alerts. It was unclear how he had managed to remain in such a sensitive post for so long.

As well as that employee’s dismissal and the resignation of the agency’s chief, Vern Miyagi, another official quit and a fourth has been suspended as a result of the incident.

According to an official report, it took nearly 40 minutes to put out the all-clear, because the agency had not rehearsed what to do in the event of a false alarm and found lines jammed by anxious callers. Efforts by Hawaii’s governor to correct the mistake were delayed because he did not know his Twitter login.

The report by the Federal Communications Commission (FCC) released on Tuesday said the error was the result of a misunderstanding among employees of Hawaii’s Emergency Management Agency during a shift change.

When he was handing over at 8am, the outgoing supervisor told the incoming day shift supervisor that he was going to conduct a ballistic missile preparedness drill.

“But there was a miscommunication,” said James Wiley, an FCC legal counsel who presented the report. “The incoming day shift supervisor thought that the midnight shift supervisor intended to conduct a drill for the midnight shift warning officers only (those ending their shift) – not for the day shift officers (those beginning their shift).”

As a result, the day shift supervisor was not in the right place to make sure that his watch officers understood it was just a drill.

Play Video 2:22 ‘This is not a drill’: Hawaiians react to 'incoming missile' false alarm – video

According to the FCC account, the night supervisor started the drill by calling the day shift warning officers, who had not been told there was to be an exercise, and pretending to be US Pacific Command.

The supervisor played a recorded message which began and ended with the words “exercise, exercise, exercise”. However, the main text of the message was not the same as that used for a routine drill, and instead followed a script used for an actual alert, including the sentence: “This is not a drill.”

Somehow, one of the day shift warning officers heard “this is not a drill”, but not the words “exercise, exercise, exercise”, and “therefore believed that the missile threat was real.”

The officer who had misheard was sitting at that terminal used to send out alerts, and chose to send a live alert from a drop-down menu.

A prompt appeared on the screen saying: “Are you sure that you want to send this alert?” and at 8.07 am, the officer clicked “yes”, sending out an all-capitals text message to mobile phones all over the state, saying: “BALLISTIC MISSILE THREAT INBOUND TO HAWAII. SEEK IMMEDIATE SHELTER. THIS IS NOT A DRILL.”

The the alert came at a time of high tensions over North Korea’s nuclear programme, heightening the ensuing panic, as people sought shelter or tried to find other family members, fearing they could be living their last moments.

“It was definitely kind of a panic zone,” Ashly Trask, who lives on the island of Kauai, said at the time. “Everyone knows you have about 15 minutes until detonation, and no one knows where it will land.”

The initial official account of the mistake suggested that a watch officer had sent a live alert by mistake by clicking the wrong online button. In its report on Tuesday, the FCC indicated it could not “fully evaluate” this new version of events as it had not been able to interview the hapless warning officer who had actually sent out the alert. The report did not say why the officer could not be reached.

The FCC report also looked at why it took so long to cancel the alert. Officials in the Hawaii Emergency Management Agency knew immediately the alert was a mistake, but it took 38 minutes for the error to be rectified on the public alert system.

Further alerts were stopped after five minutes, but the cancellationdoes not automatically generate an “all clear” message, or recall the earlier false alerts. A further 13 minutes was spent trying to get through to radio and TV stations to inform them of the error.

Then a Facebook and Twitter message was put up on the emergency management agency’s accounts. It was not until 8.24am that the corrective message was retweeted by the Hawaii governor, David Ige.

The FCC report notes drily: “The governor has stated that he was unable to do this earlier because he did not know his Twitter password.”

It was not until 8.27 – 20 minutes after the false alarm – that an agency meeting was held to discuss putting out a correction on the statewide emergency alert system, but it took from 8.31am to 8.45 to log on to the appropriate software, create the correction and then send it out, in part because there was no template for a false alert message.

Missile defence drills have been suspended until a full inquiry is completed.