PARIS (AP) — Four people were arrested Monday in a sweep against French jihadist recruiters, the country’s top security official said a day after authorities announced the detention of a French suspect in the deadly shooting at a Jewish museum.

The arrests come as investigators question a suspected French jihadist who had spent time in Syria. Mehdi Nemmouche was arrested Friday in connection with the death of three people at the Belgian museum, carrying firearms, ammunition and a video claiming responsibility for the May 24 attack. French Interior Minister Bernard Cazeneuve said that Nemmouche was arrested minutes after he set foot on French soil during a customs check.

Western governments are increasingly alarmed about fighters returning from Syria — many have come back radicalized and with newfound knowledge and weapons.

The numbers shift constantly — people cross the border freely from Turkey and, with European passports, return home just as easily. Their journey to Syria itself is getting simpler, as networks of recruiters take care of travel arrangements and even training once potential fighters cross into the civil war-torn country.

The arrests Monday, which prosecutors said were not linked to the Jewish museum shootings, were one such network that Cazeneuve said operated in in the Paris region and the south of France.

“We will not give terrorists a chance,” Cazeneuve told Europe 1 radio.

Between 1,000 and 1,500 Europeans may be currently fighting in Syria against President Bashar Assad, according to Charles Lister, an analyst with Brookings Doha Center, who drew the estimate from governments and other sources.

It is not yet known whether the attack on the Jewish museum was ordered from within Syria or was simply an individual act. So far, there has been no evidence that the radical Muslim rebel groups in Syria are looking to expand their battle.

“The most important question is whether he was working as a lone wolf or whether he was working under an organization in the Middle East,” Lister said.

French prosecutors said Nemmouche had not spoken to investigators yet.

In a one-minute rampage that deeply shook Europe’s Jewish community, a gunman opened fire at the Brussels museum. In addition to the fatalities, another person was gravely wounded.

Authorities raised anti-terror alert levels as they searched for the attacker. But it was ultimately a customs inspection in the French port city of Marseille that turned up Nemmouche, as he disembarked from a bus coming from Amsterdam, according to Paris prosecutor Francois Molins.

Molins said the suspect, a French-born 29-year-old from the northern city of Roubaix, had a revolver and a retractable automatic weapon like those used in the Brussels attack, and ballistics analyses were underway to determine if they were the same weapons.

At least one of the weapons was wrapped up in a white sheet scrawled with the name of the Islamic State of Iraq and the Levant, an extremist group fighting in Syria, Molins said. The group has also waged attacks in Iraq.

Two of Nemmouche’s acquaintances were interviewed and released, Belgium prosecutor Frederic Van Leeuw said Monday.

The Brussels killings led Belgian officials to boost their anti-terror measures, and raised fears of rising anti-Semitism. Cazeneuve said France had already raised its anti-terror measures, and notably had increased security at Jewish sites.

In 2012, a French-born radical Muslim attacked a Jewish school during a rampage in the south that left seven people dead. The attacker, Mohammed Merah, was killed in a shootout with law enforcement.

“I understand that Jews in France are worried,” Cazeneuve said.

Late last month, French prosecutors opened a terrorism probe into Merah’s sister and raided her home in the southern city of Toulouse. She was not there — authorities believe she fled to Syria.

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Follow Lori Hinnant at: https://twitter.com/lhinnant

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Associated Press writer Raf Casert contributed from Brussels.