A Frenchman has been arrested at a train station in Marseille over a fatal shooting at the Jewish Museum in Brussels eight days ago.
Mehdi Nemmouche, 29, is being held on suspicion of murder and attempted murder. He was reportedly carrying a Kalashnikov rifle and a handgun similar to the ones used in the May 24 attack.
Three people were killed while a fourth victim is critically injured.
Security was tightened at Jewish sites across Belgium following the attack.
Mr Nemmouche, a French national who is believed to be from the northern town of Roubaix, is also suspected of having been with Islamists militants in Syria last year.
French President Francois Hollande confirmed a man had been arrested and said France was determined to stop “jihadists” from carrying out attacks.
“We will monitor those jihadists and make sure that when they come back from a fight that is not theirs, and that is definitely not ours… they cannot do any harm,” he told reporters.
Mr Nemmouche was arrested during a customs check at the Saint-Charles train station in the southern French city of Marseille on Friday.
He had been on board a bus that was travelling from Amsterdam via Brussels.
An official at the Paris prosecutor’s office told the Associated Press that ballistics tests would be carried out to determine whether the weapons he was carrying were the same as those used in the Brussels attack.
Three people were killed outright when a gunman opened fire at the museum in the busy Sablon area of the Belgian capital. They were an Israeli couple in their 50s, and a French female volunteer.
A Belgian man, believed to be an employee of the museum, was critically injured.
The Belgian prosecutor’s office said the victims were struck by bullets in the face or throat, in what Brussels Mayor Yvan Mayeur said was probably a “terrorist act”.
One person was detained after he drove away from the scene around the time of the attack, but he was later released.
Belgian Foreign Minister Didier Reynders, who was one of the first people to arrive at the scene, said: “You cannot help think that when we see a Jewish museum, you think of an anti-Semitic act. But the investigation will have to show the causes.”
Israeli Prime Minister Benjamin Netanyahu and President Hollande also condemned the killings.
Belgium has a Jewish population of some 42,000, about half of whom live in the capital.