Authorities in Vienna now believe shooter who killed 4 likely acted alone

Authorities are now saying that the gunman who killed four and wounded 22 in Vienna on Monday night probably acted alone.

Shots rang out in the center of town around 8 p.m., near a synagogue, a spokesman for the Ministry of the Interior told ABC News. The shooter appears to have opened fire in five other places downtown, the spokesman added.

The attack happened on a crowded night in Vienna -- the eve of a nationwide lockdown taking effect -- and many people barricaded themselves in restaurants and concert halls for hours as police scoured the area for suspects. As of midnight, police still had not ruled out a second shooter, and authorities still have not ruled that out entirely.

Authorities are about halfway done combing through tens of thousands of submitted photos and video clips tied to the incident. Those findings are expected to be reported in full at a Wednesday afternoon press conference.

Austrian Interior Minister Karl Nehammer said at a Tuesday press conference that the suspect was shot dead by police about nine minutes after the first emergency call reached authorities.

Nehammer confirmed additional details about the attacker, who ran amok through central Vienna brandishing an automatic rifle, a pistol, a fake explosive vest and a machete.

The attacker was identified as 20-year-old Kujtim Fejzulai, a dual Austrian-North Macedonian citizen with connections to the Islamic State. He had been sentenced to 22 months in prison on April 25, 2019, on terrorism charges -- he'd attempted to travel to Syria to join ISIS. But because he was a minor, he was released on Dec. 5.

"The fact is that the terrorist managed to deceive the judicial system's deradicalization program" in securing his release, said Nehammer, who added that the system should be reevaluated.

In the early hours of Tuesday morning, Austrian special forces raided 18 homes of people with connections to the shooter and took 14 individuals into custody for questioning, authorities said at the press conference.

Later on Tuesday, an 18-year-old man and a 24-year-old man were arrested near Zurich in connection with the shooting rampage.

"The extent to which there was a connection between the two arrested persons and the alleged assassin is currently the subject of ongoing clarifications and investigations," Swiss police said in a statement.

The Austrian government announced a three-day mourning period, during which flags will be flown at half-staff, and at noon on Tuesday a moment of silence was observed.

"Yesterday's attack was clearly an Islamist terror attack," Chancellor Sebastian Kurz said. "It was an attack out of hatred -- hatred for our fundamental values, hatred for our way of life, hatred for our democracy in which all people have equal rights and dignity."

After a tense night, the Austrian capital came to a virtual standstill on Tuesday. Authorities encouraged children not to go to school, although schools still were open amid the lockdown. While it remains unclear whether the synagogue or local Jewish community specifically were targeted, all Jewish houses of worship, kosher restaurants and supermarkets remained closed across the nation.

World leaders -- President Donald Trump, Joe Biden, Emanual Macron, Angela Merkel and Pope Francis among them -- voiced their support and condolences in the wake of the attack.

"Enough violence! Let us together strengthen peace and fraternity," the pope said via Twitter. "Only love can silence hate."

ABC News' Kirit Radia contributed to this report.