- UK Prime Minister's office says number of Britons killed likely to grow substantially
- Attack is likely the deadliest terrorist assault on Britons since 2005 London transport bombings
- Major anti-terrorism exercise to be held in UK in coming days
(CNN)The scenes were horrific: Vacationers snapped from their naps by the sound of gunfire. The screams of the injured. Blood on the beach.
The attack Friday at a Tunisian beach resort killed at least 18 Britons, but officials said Monday the actual number was likely to be 30. That would make it by far the deadliest terrorist attack on Britons since the London transport bombings 10 years ago.
Yet it was just one of three terrorist attacks on three continents that followed one another in frightening succession last week. While it is not yet known whether the attacks were linked, they have left officials in numerous countries concerned about how to keep their citizens safe.
A series of terrifying attacks
-- On Friday morning, European time, the head of the decapitated manager of a transportation company was found hanging from a fence near Lyon, in southern France, along with two banners said to bear Islamic writing. The attack, at a U.S.-owned factory, began when a delivery van, allegedly driven by Yassin Salhi, entered the facility, according to Paris Prosecutor Francois Molins, whose office handles anti-terrorism cases throughout France.
-- Not long after that, a bomb ripped through Al-Sadiq mosque in Kuwait's capital during Friday prayers -- a time when mosques are packed with worshippers. At least 27 people were killed. ISIS has posted a video of a man it claims is the suicide bomber, identifying him as Abu Suleiman al-Muwahid -- presumably a nom de guerre.
-- Then came news that a gunman had sprayed a tourist beach in Sousse, Tunisia, with bullets, killing at least 38 people and injuring at least 39 others. ISIS has claimed responsibility for that attack, as well, though this claim may be more tenuous.
The attacker, identified by authorities as 24-year-old Saif Al-Deen Al Rezgui, was shot dead by police Friday outside the hotel.
Tunisian Interior Minister Najem Gharsalli said Monday that the country had made its first arrests in connection with Friday's attack.
"We have started by arresting a first group, the important part of the network that was behind this terrorist criminal," Gharsalli said. He made the announcement during a joint press conference with his German, French and British counterparts in Sousse.
Was Tunisian attacker associated with terror group?
Tunisian Prime Minister Habib Essid told CNN's Becky Anderson Monday that there was little information on whether Al Rezgui was involved in an organized terror group.
Studying at university, the gunman was involved in a few organizations on campus that are being investigated for ties to the attack.
"We cannot establish at this moment, but there is some information that he belonged to an organization," he said, describing a mosque that may have influenced Al Rezgui's thinking.
A spokesman for Tunisia's Interior Ministry told CNN that Al Rezgui indeed had a connection to a Libyan terrorist organization. However, Mohammed Ali Aroui said it was unclear whether the gunman had a link to the Ansar Al-Sharia militant group or ISIS in Libya.
Police have also not yet confirmed whether Al Rezgui had ever traveled to Libya, he added.
Al Rezgui had a Tunisian passport but it's unclear whether he traveled out of the country for training with a militant group. Essid noted that sometimes people illegally cross the border between Libya and Tunisia.
Essid added that Al Rezgui could have used social media and online tools to prepare him for attack.
With Tunisia being such a popular tourist destination, the massacre has brought a wave of devastation over the economy.
"It's a heavy damage because the sector is drowning. We must do everything to save the situation," Essid said.
A terrible toll for Britain
For Britain, the news coming from Tunisia is likely to get worse.
A spokeswoman for British Prime Minister David Cameron said Monday that 18 British nationals were known to have been killed during the attack Friday. But the spokeswoman said the grim toll would, in all probability, increase to about 30.
The interior ministers of Britain, Germany and France gathered Monday in Tunisia to show their solidarity with Tunisian authorities and to vow to defeat the terrorists.
Theresa May, Britain's home secretary, said all were united in their determination "to fight against this perverted ideology that is causing this death and destruction."
The ministers said their meeting would be followed by other meetings of experts from their various countries.
Gharsalli, the Tunisian interior minister, said his government might close mosques "that use an anti-democratic language and that use a language of hatred."
In Britain, Cameron told the House of Commons that a major counterterrorism training exercise would take place in London over the next few days. The exercises will involve security and emergency services, he said.
A national minute of silence -- a rare event in Britain -- will be held this Friday, a week after the Tunisia attack occurred, Cameron said.