It is unclear who was behind Friday's hoaxes.

The emails to RTE and Nova were signed off with the name Mikhail Andropov, ‌with a message saying the author was one of the leaders ‌of a group responsible for this week's plane hoaxes.

"But this is only the beginning. What happened this week is just a small part of what awaits Europe in the near future," the message added.

The purported author shared a surname with Yuri ⁠Andropov, a former General Secretary of the Communist Party of the Soviet Union. The title of the email “MESSAGE TO THE WEST,” echoed the title of a biography of Andropov called “New Challenge to the West”.

It was unclear if this was a sign of Russian involvement, or an attempt to point the finger at Moscow.

Police came to the Nova newsroom in Sofia, which was evacuated, checked and deemed safe. Broadcasting was not interrupted and employees returned to their desks, broadcasters said.

Staff returned to RTE's campus after police completed ‌a more than three-hour sweep of the buildings.

Irish prime minister Micheal Martin said security agencies in ​Europe had not yet determined whether the threats to air travel and media organisations ‌were being made by individuals or as part ⁠of a coordinated attack.

"It just illustrates how relatively simple it can be to cause disruption ⁠and to undermine the normal operation of society through unconventional activities of this kind, which are designed to cause disruption, designed to ‌cause fear and anxiety," Martin ​told RTE radio shortly after news programming went back ‌on air from an outside broadcasting unit.

"We ​have to remain calm," he added.

Bomb threats warning "the clock is ticking" were sent to TV stations across Europe on Friday, prompting evacuations and programming to be suspended.

Warnings were sent to Irish broadcaster RTE, Britain's Sky, Bulgarian private station Nova TV ⁠and others, the companies said. No bombs were found at any affected site. RTE programmes were replaced by music and a pre-recorded message, some for as long as three hours.

The alerts follow a string of hoax bomb threats that has disrupted passenger flights from Serbia to Finland to Spain and beyond in recent days, forcing diversions, emergency ⁠landings and security sweeps, though no explosives have been found ​so far.

They ⁠also come amid a growing sense of unease about hacking scams in recent weeks, including cyberattacks on supermarkets.

"We want the whole world to know about us, which is ⁠why one of our operatives has planted an explosive device in your building," read one email received ​by ⁠Nova and RTE on Friday. "It will detonate ‌in 45 minutes. The clock is ticking."

Jesper Frederiksen, Heimdal's CEO, tells The Independent that the recent attacks on fridges, blood banks, and hoaxes on aviation all line up with “psychological and political” forms of hybrid warfare.

“There’s no money in this, so the payoff is psychological and political. These attacks prove reach, test how fast governments respond, and wear down public confidence that the state can protect everyday life.

“Food, blood and air travel were chosen because everyone depends on them and the disruption is instantly visible.”

Read below the full statement from the Taoiseach to the bomb threats made against Irish media organisations:

“The threats made this morning against media organisations here are very serious.

“The Gardaí were alerted to these threats and immediately took the precautionary actions that were necessary in co-operation with those who have been targeted.

“It is important to emphasise that the first concern in incidents such as these must be the safety and security of the people who are affected.

“Hoax threats such as these cause fear, distress and disruption to people going about their daily lives and activities; they achieve no positive purpose whatsoever.

“It is not known yet who is responsible for these threats, whether it is individuals or a centralised or co-ordinated effort. However, these threats come on the heels of the ongoing disruption to aviation around Europe caused by similar threats, and it seems likely that this is a co-ordinated campaign of disruption.

“The Gardaí, working in cooperation with the Defence Forces, and our cyber and aviation authorities, are working to establish who is responsible. The Gardaí and the Defence Forces are in ongoing liaison with their European security counterparts. Every effort will be made to identify those responsible and to hold them to account.

“The main ⁠priority, however, is to ensure that people are kept safe, despite the unavoidable and unwelcome disruption that this may cause.

“The Gardaí have been alive to the risk of disruptive threats such as these in the context of planning for EU Presidency events and are exercising the relevant protocols in response.”

RTÉ staff including radio and television presenters in Dublin were told to leave the studio buildings after an email was received on Friday morning.

Children being looked after in a creche on site were also evacuated.

The incidents came after bomb threats were sent to airlines across Europe, including one targeting flights from Dublin, over recent days.