JERUSALEM — Almost 20 years after Israel was shaken by the exposure of a Jewish underground in the West Bank, some fear that another one could be emerging.

Recently, police detained four Jews from settlements in the Hebron Hills region suspected of conspiring to plant a booby-trapped cart in a Palestinian girls' school in Jerusalem.

Earlier this week, police arrested two more suspects, Noam Federman and Menashe Levinger, in connection with the alleged plot. Federman is a widely known anti-Arab activist.

If the incident does prove to be part of a larger movement, which was initially exposed by the General Security Service, it could complicate a tentative opening toward peace-making and undercut public support for the embattled settlement movement.

This latest incident, however, was detected by chance, when a regular police patrol spotted a van like the type frequently used by the settlers as security vehicles, stopping near the Palestinian Al-Mukassid Hospital.

The passengers tried to detach from the car a cart laden with explosives. It reportedly was set to explode at 7:35 a.m., when the street would be humming with girls on their way to school.

Police detained Shlomo Dvir (Seliger), 27, of the Bat Ayin Bet settlement in the Hebron Hills; Yarden Morag, 25, of Bat Ayin; Ofer Gamliel, 42, also of Bat Ayin; and Yosef Ben-Baruch, 23, of the Maon Farm, also near Hebron.

The four are suspected of plotting to carry out a terrorist attack. They denied the charges, and said they were framed and tortured by their investigators.

A Jerusalem court extended their remand.

It was the first time since the intifada began in September 2000 that security forces have laid their hands on Jews suspected of plotting terror attacks against Palestinians. The incident could have been overlooked as a marginal event, but there are increasing signs of renewed Jewish vigilantism in the West Bank.

Seven Palestinians have been killed in suspected terror attacks against Palestinians in the past two years, and 10 have been wounded.

Police have not cracked any of the cases but are checking whether the four settlers from the Hebron region are associated with previous attacks.

The settlement establishment was shocked by the arrests. Yesha, a group representing Jewish settlers, issued a statement sharply condemning the alleged plot.

When the Jewish underground was exposed in the 1980s, Yisrael Harel, then Yesha's chairman, was one of its most outspoken critics — although some underground members were among his best friends.

However, Harel said he doubts the four men detained in the incident at the girls' school had done anything similar to the subversive activities by members of the 1980s underground.

"I do not have any inside information," said Harel, a resident of Ofra in the West Bank, "but I suspect that even if these people are guilty, they represent only themselves."

Most Hebron Hills residents have not shown any exceptional militancy against the Palestinians, Harel said.

That, of course, is a matter of political perception. Legislator Yossi Sarid, the head of the Meretz Party, said after the arrest that the incident could be another example of the weeds of terrorism growing in the settlements.

Jibril Rajoub, head of the Palestinian Preventive Security Service, questioned recently why Israeli security forces were so effective at nabbing Palestinian terrorists, but were unable to expose Jewish ones.

The answer, of course, is that thousands of Palestinians are engaged in violence, as compared with a handful of Jewish militants.