BEIRUT — Lebanese authorities arrested five Syrians and a Palestinian suspected of being implicated in the twin suicide bombings in central Beirut on Thursday that killed 43 people, a senior security source said on Saturday.

The bombings in a busy residential and commercial area that is a stronghold for Lebanese Shi’ite movement Hezbollah were claimed by Islamic State.

"Within 24 hours the network was arrested in the fastest uncovering of a bombing incident in the country," the source said. The explosions were the first attacks in more than a year to target a Hezbollah stronghold in Lebanon, as the Iran-backed group steps up its involvement in the war in neighboring Syria

Islamic State said in a statement posted online by its supporters that its members blew up a bike loaded with explosives in Borj al-Barajneh and that when onlookers gathered, a suicide bomber blew himself up among them. The group said the attacks killed 40 people.

On Friday, Lebanon held national day of mourning for the twin explosions. Witnesses at the scene say the area was targeted because of its predominately Shi’ite population.

"They targeted this place because they don’t have any other way to fight us," Fouad Khaddam, a witness at the scene, told Al Jazeera. "They have run out of options … They targeted this area because we are Shia. But let me be clear: We won’t be fazed."

Hezbollah has sent hundreds of fighters to support Syrian President Bashar al-Assad’s forces in the four-year-old conflict over the border.

Government forces backed by Hezbollah and Iranian troops have intensified their fight against mostly Sunni insurgents, including Islamic State, since Russia launched an air campaign in support of Assad on Sept. 30.

"This is probably just to remind Hezbollah there are other [groups] who can take revenge," Lebanese political analyst Kamel Wazne told Al Jazeera. "It might be again the beginning of a circle of violence for Beirut."