HALLE, Germany — In a hate-filled screed written in English and published online before he tried to breach a synagogue in eastern Germany, the gunman who killed two people made clear not only that he had chosen his target hoping to kill as many Jews as possible but also that he hoped to impress a wider audience.
The police made an arrest soon after the shooting. The man detained was identified on Thursday only as Stephan B., a German citizen. He is suspected of having written the manifesto and of having carried out the attack, which took place on Yom Kippur, the holiest day on the Jewish calendar. Further information was expected in the course of the day on Thursday.
Those who were worshiping inside the Humboldt Street synagogue said on Thursday that the congregation had remained calm throughout the attack. But some questioned why no police guard had been assigned, as requested by their cantor, and why it had taken officers what they estimated to be 10 minutes to respond.
The attack was thwarted by a locked, heavy wooden door — the two victims were killed on the street outside and in a nearby kebab shop. On the security camera screen, Max Privorozki, the head of the congregation, said he could see the fuzzy images of the heavily armed attacker on the other side.
“It was a miracle that the door held,” Mr. Privorozki said in an interview on Thursday. “I cannot imagine what would have happened if it had not.’’
Vigils of solidarity with the Jewish community and mourning for the victims took place in Berlin, in Halle and in the nearby city of Leipzig, where people had gathered to commemorate the 30th anniversary of the street protests in the former East Germany that led to the fall of the Berlin Wall and the collapse of communism across Eastern Europe.
In the upheaval of the 1990s, Germany saw a rise in far-right extremism and the targeting of minorities and refugees from countries in the Middle East. But experts said that the manifesto and the live-stream of the attack on Wednesday indicated that the gunman had been influenced by other recent assaults by far-right extremists abroad, such as those on two mosques in Christchurch, New Zealand.
“If I fail and die but kill a single Jew, it was worth it,” the attacker wrote in the manifesto that was found by researchers at the International Center for the Study of Radicalization and Political Violence, a research organization at King’s College London. “After all, if every White Man kills just one, we win.”
The gunman’s manifesto, which included detailed descriptions of the self-made weapons that he used, also included thoughts about the merits of targeting Jews over Muslims, whom he appeared also to despise.
That it was written in English also indicated that the attacker was seeking to draw the attention of an audience wider than just other extremists in Germany, said Peter R. Neumann, a professor of security studies at King’s College London and director of the center for the study of radicalization.
“It clearly shows that he wasn’t trying to impress local neo-Nazis, but that his ‘audience’ was on message forums like 8chan and his heroes were people like Anders Breivik and the attackers in New Zealand and El Paso,” Mr. Neumann said.
The gunman did not appear to have been on the radar of the German security services. Mr. Neumann added that a shift to a more globalized, digitized version of right-wing extremism could prove challenging in Germany, where officials were familiar with local neo-Nazis networks but have been slower to respond to developments online.
“I think they are less good when it comes to message forums like 4chan and 8chan and this new, more diffuse and ideologically more promiscuous far-right extremism,” Mr. Neumann said.
Jewish leaders were demanding to know why their appeals for increased police presence around the synagogue were ignored. While Jewish institutions in most large cities in Germany have a round-the-clock police detail, that was absent in Halle.
Mr. Privorozki said that the heavy wooden door of the synagogue and the community center’s electronic security system had recently been upgraded with the help of the nonprofit Jewish Agency for Israel.
He said that the police had taken some time to respond to the call. “I did not look at my watch, but I guess it took at least 10 minutes,” Mr. Privorozki said, adding that the police had promised to always be close to the synagogue and to come quickly in an emergency.
Congregants described the atmosphere inside the synagogue as calm, even during the attack.
“We felt relatively safe because our security man and the cantor had taken all the precautionary measures,” said Shimon Meyer, who attended with his wife, Luba. “We stayed quiet and collected — there was no trace of panic.”
The congregation was moved to a safer part of the synagogue once the police arrived, but continued the service.
Ms. Meyer said, “It took a while before the police came, but when we could hear the sirens and a helicopter, we continued a prayer and sang.”
When worshipers were finally escorted out of the synagogue and onto a bus, they continued their songs, surprising the bus driver, Ms. Meyer said. At the hospital, they finished the service, even blowing the shofar, or ram’s horn, to mark the end of Yom Kippur at sundown, she added.
“We wanted to show that we weren’t defeated,” she said, “But that we defeated the situation.”