German police arrested four people this morning after an investigation into an Islamist-motivated threat situation on Yom Kippur, Judaism’s holiest day. Two years ago a deadly attack in another German city happened on Yom Kippur. A teenager, a Syrian national who lives in Hagen, was detained on Thursday morning while three other people were arrested after a raid on an apartment. Police cordoned off a synagogue in Hagen on Wednesday and an evening worship service was called off.

The arrests happened after police received a tip.

Herbert Reul, interior minister of North Rhine-Westphalia, said officials had received “very serious and concrete information” that there could be an attack on the synagogue during Yom Kippur.

The tip, he said, had pointed to “an Islamist-motivated threat situation” and named both the possible timing and would-be perpetrators.

Police are still investigating and searches are ongoing. It is not yet clear that all of the four were involved in the terror plot. Police brought sniffer dogs to the synagogue but no traces of explosives were detected. German police contacted the local Jewish community as they investigated. Dortmund police spokesman Gunnar Wortmann told AP: “We had information that there was a threat to a Jewish institution here in Hagen.” The German Justice minister released a statement Thursday.

German Justice Minister Christine Lambrecht sharply condemned the foiled Hagen attack on Thursday. “It is intolerable that Jews are again exposed to such a horrible threat,” she said, “and that they cannot celebrate the start of their highest holiday, Yom Kippur, together.”

There is a piece in Der Spiegel today that explains the investigation and reminds readers that just two years ago a young extremist radicalized himself online without detection from authorities. Security authorities were criticized for being “blind, slow, and careless” then. The teenager is accused of planning an explosive attack on the synagogue in Hagen. He was arrested in front of the main train station in Hagen. Officials are evaluating electronics and cell phones, though no explosives have been found.

According to SPIEGEL information, it was a warning from a foreign secret service that made the German counter-terrorism humming as if at the push of a button. It was a lesson in international cooperation between police and intelligence services. And yet the case shows once again how dependent Germany continues to be on the powerful services of friendly countries when it comes to security issues.

On Wednesday afternoon, a few hours before the start of the celebrations for the Jewish holiday of Yom Kippur, the partner service passed on its findings to the Federal Intelligence Service ( BND ) via monitored communication on the Internet. A suspected Islamist, according to the warning, was communicating with a chat partner via an explosives attack. Crime scene, perpetrator, time of the crime – according to security circles, the information was as detailed as it is rarely the case.

The BND immediately shared the information with all relevant authorities. A short time later, a large group of police secured the synagogue, and explosives detection dogs combed the area.

During the night, the trail led to the 16-year-old Syrian who lives with his father and two brothers in an apartment on the first floor of an apartment building in downtown Hagen. About an hour and a half after his arrest in front of the Hagen train station in the morning, a special task force stormed the apartment. The police initially took the rest of the family into custody.

The teenager came to Germany through Beirut in the spring of 2015 as a part of family reunification. The father was already living in Hagen as an asylum seeker for the past few months. He was recognized as a refugee not long later. Apparently, the family lived quietly and were very poor. The teen was not known to have been an Islamist or to have connections to the local community. He likely radicalized himself online, as the other terrorist did before the attack two years ago. The latest reports are that he is still at police headquarters undergoing questioning. One politician running to succeed Angela Merkel as the chancellor said much the same as Herbert Reul did. “It appears that prior to today on Yom Kippur, a Islamist motivated attack was averted,” said Armin Laschet, the state premier of Germany’s most populous region North Rhine-Westphalia, where Hagen is located.”We will do everything we can to clarify which networks may have been behind” the plot, added Laschet.

A terror attack on a synagogue on the holiest day of the Jewish calendar was stopped by security officials who learned the lesson from two years ago. They investigated a tip and it lead them to the 16-year-old teen. It’s unclear if the rest of his family was involved. I’d be willing to bet he’s a lone wolf who became radicalized online, a tool that radical Islamic terrorists have perfected. Their techniques are particularly appealing to young people.