A suicide plot was intended to strike an Armed Forces Day parade, targeting troops who had fought in Afghanistan and Iraq, a British newspaper reported.
Abdulrahman Sabah Eidan Saud was found hiding at an extremist's house.
British Prime Minister David Cameron says those responsible for the attack in Tunisia "only unite us more strongly in our determination to defeat these Islamist extremists."
The terrorist attack Friday claimed at least 39 lives, with Belgian, British and German nationals among the dead.
Authorities have announced that 80 mosques in the country will be closed for promoting terrorism, a day after at least 39 people were killed in an alleged ISIS attack at a beach hotel.
Department of Homeland Security Secretary Jeh Johnson urged law enforcement to "remain vigilant" as Independence Day approaches.
At least one gunman attacked a luxury hotel Friday in coastal Sousse, Tunisia, leaving scores of beachgoers dead, the country's Interior Ministry said.
The accident took place outside the town of Fahs -- approximately 60 miles west of the capital city of Tunis -- early on Tuesday.
An unidentified Libyan group attacked the Tunisian consulate in Tripoli Friday, amid an ISIS expansion in the area.
Nigerian President Buhari requested assistance from the G7 leaders on Nigeria's chief concerns including terrorism, economy, infrastructure and corruption.
Merkel, once dubbed the "climate chancellor," hopes to revitalize her green credentials.
The soldier who carried out the attack on his fellow troops in Tunis was reportedly not allowed to carry a weapon.
The 22-year-old Moroccan man, identified as Touil Abdel Majeed, had arrived in the Sicilian port of Porto Empedocle along with 90 other migrants.
Prime Minister Shinzo Abe is increasingly focusing on the expansion of military, but his policies have been controversial among the public.
In a rare audio statement released Thursday, ISIS leader Abu Bakr al-Baghdadi called on all Muslims to fight a war in which the "Islamic State is merely the spearhead."
There are about 1.6 million Muslims living in Spain, representing about 3 percent of the population. Many hail from nearby Morocco, Algeria or Tunisia.
The warning comes less than two months after 21 people were killed in a Tunis museum.
A top U.N. human rights official singled out a Sun newspaper column that compared migrants from Africa to cockroaches.
The crackdown on people smugglers advocated by various European officials will be difficult to implement.
Syria will celebrate 69 years of independence from foreign rule on Thursday, but the Arab country is still besieged by a civil war.
The move comes as Tunisia struggles to secure its porous borders with Algeria and Libya that are used by militants and smugglers.
According to the report, a large number of foreign fighters were from Tunisia, Morocco, France and Russia.