WASHINGTON — (CNN) The Syrian suicide bomber who injured 15 people in a Sunday night attack on a music festival in Ansbach, Germany, left behind a video pledging allegiance to ISIS leader Abu Bakr al-Baghdadi, Bavarian Interior Minister Joachim Herrmann said Monday.

The ISIS-affiliated media group Amaq also claimed responsibility for the attack.

In the video left on a mobile phone, the 27-year-old Syrian announced his threat to carry out an attack “as an act of revenge against Germans, because they obstruct Islam,” Herrmann told a press conference.

The man said the attack would be committed in the name of Allah as retaliation for the killing of Muslims.

The bomber, who has not been named, was killed.

“I believe that after this video we cannot doubt that this attack was an Islamist terror attack,” Herrmann said.

Fifteen people were injured in Sunday night’s blast in Ansbach, four of them seriously, Ansbach Mayor Carla Seidel said at a news conference Monday.

The attack has further rattled the German public following a week of violence in southern Germany that began July 18, when an immigrant teen, apparently inspired by ISIS, stabbed passengers on a train in Wurzburg in Bavaria.

That was followed by a shooting spree Friday in which nine people were killed in the Bavarian state capital, Munich, before a Syrian asylum seeker killed a woman in Reutlingen, in the neighboring southern German state of Baden-Wurttemberg, on Sunday, hours ahead of the Ansbach attack.

The wave of violence has fueled criticism of Chancellor Angela Merkel’s welcoming stance toward immigrants, with the hashtag #Merkelsommer, or “Merkel summer,” circulating on social media.

Germany accepted more than 1 million asylum seekers last year, and some Germans have expressed fears that terrorists might have entered the country among them, or that disaffected youths among the refugees could be susceptible to radicalization.

The attacker, who has not been named, was a rejected asylum seeker known for petty criminal offenses who was slated to be deported to Bulgaria, officials said Monday.

German Interior Thomas de Maiziere said the attacker arrived in Germany two years ago and applied for asylum in 2014.

The request was denied, and under the auspices of the Dublin Regulation — a European Union law governing states’ responsibility for asylum seekers — the attacker was supposed to be deported to Bulgaria, the country where he had entered the EU, said Johannes Dimroth, spokesman for Germany’s Federal Interior Ministry.

However, the deportation was temporarily suspended due to medical evaluations, and the man was placed in a psychiatric clinic at some point, de Maiziere said. Officials said he had twice attempted suicide before the bombing.

Police searching his home, at a hotel used as an asylum shelter, had found two mobile phones and multiple SIM cards as well as material to make explosive devices, Herrmann said.

The bombing took place at the final concert of the Ansbach Open music festival Sunday night, where about 2,500 were in attendance, police said.

At about 9:45 p.m. Sunday, security staff at the concert noticed a suspicious person wearing a backpack pacing up and down around the entrance to the event, a police statement said.

The man had been denied entry into the concert since he did not have a ticket, according to Bavarian Interior Minister Joachim Herrmann.

The police statement said that the bomber “lingered around in the outdoor seating area of a nearby wine restaurant.”

“An explosion took place at around 10:10 p.m. in that area after the young man briefly leaned forward, according to eye witness accounts,” it said.

The music festival and surrounding area were then evacuated.

Herrmann said the attacker’s backpack contained screws and nails in an apparent bid to inflict widespread damage.

Officials opened the crime scene Monday, revealing beer glasses abandoned at tables, playing cards flecked with blood and a chalk outline of where the bomber’s body was found.

Ansbach, with about 40,000 people, is a major U.S. military garrison town, with around 5,000 members of the military living there along with civilians, contractors and retirees. There are three military installations in the Ansbach area, according to the garrison’s website.

The Ansbach bombing is the fourth attack in southern Germany — and the third in the state of Bavaria — in recent days, which also came on the heels of the Bastille Day ISIS attack in Nice, France, that killed 84 people.

Speaking at a press conference Monday, Hermann, the interior minister, acknowledged it had been a “very terrible week” in Bavaria.

“Yes, this was also for me personally a very terrible week, as I think it was for most of the people in Bavaria.

The attack last Monday on the train in Wurzburg, then the rampage … in Munich Friday night, and now again an attack.

“It has been almost nine years now that I am interior minister.

And I have not had, thank goodness, to experience something like this during these nine years until now.

And I do hope that I will not experience something like that so soon and fast.”

A week ago, a 17-year-old who had arrived in Germany as an unaccompanied minor from Pakistan or Afghanistan carried out a stabbing attack on a train in Wurzburg, about 70 kilometers (45 miles) from Ansbach.

The attack, which authorities said appeared motivated by ISIS propaganda, has left four people hospitalized, including one in an induced coma, medical officials said.

On Friday, an 18-year-old with dual German and Iranian nationality went on a shooting spree in a busy shopping district in the Bavarian capital, Munich, killing nine people before killing himself.

Police said the gunman was a mentally troubled individual who was obsessed with mass shootings and may have planned the attack for a year. Authorities have not found a link to terror groups.

And on Sunday, hours before the Ansbach attack, a 21-year-old Syrian asylum seeker killed a woman with a machete in the city of Reutlingen.

The attacker had been granted asylum in October after passing security checks, said de Maiziere, although police said he was known to authorities for property thefts and assault.

The woman was 45 and from Poland, police said.

Police said the deadliest attack — the Munich rampage — was carried out by a German-born teen who had no apparent links to Islamist terror or the recent wave of migration, but the violence has fueled public concerns about the security ramifications of Germany’s welcoming stance toward migrants entering Europe, the majority of them from Muslim countries.

Germans were shocked by sexual assaults of women blamed on immigrants at New Year’s Eve festivities in Cologne and other cities, and three Syrian men were arrested last month on suspicions they were planning to carry out a mass casualty attack in Dusseldorf.

Police announced Monday the arrest of a 16-year-old Afghan national who knew the Munich gunman, and had met with him shortly before the rampage, officials said.

Chief prosecutor Thomas Steinkraus-Koch said that, when police interviewed the teen Friday night in the aftermath of the attack, he deleted a WhatsApp chat log of his conversations with the attacker.

But police have since been able to recover the chat log, which showed the teen knew the gunman had munitions, leading authorities to suspect the teen may have known about the attack ahead of time.

The two teens, both loners who played a lot of first-person shooter games, had met while receiving inpatient psychiatric care last summer and had exchanged messages discussing their fantasies of going on rampages, officials said.

The 16-year-old had known that the gunman was obsessed with Anders Behring Breivik, who killed 77 people in Norway in 2011.

The Munich attack was carried out five years to the day since Breivik’s rampage.

Hermann Utz of the Munich criminal investigation department said the 16-year-old’s apartment had been searched.

The teen will appear before a judge Monday.