Turkish police have announced four swift detentions connected with Tuesday’s car bomb attack on a police bus in Istanbul. 11 people are confirmed dead, including seven police officers and four civilians, while at least 36 more were injured, several of them critically.

“A bomb-laden car was detonated at around 8:35 a.m. as a police bus was passing near a police station, Istanbul Gov. Vasip Şahin told reporters, in the third major bombing in the country’s biggest city in 2016,” reports Hurriyet Daily News.

Sahin added that the attack “targeted vehicles carrying members of a riot police unit.” The unit was en route to Istanbul University for security duty on a day of exams, which have now been canceled.

The attack was carried out using a rented car rigged to detonate with a remote control. The four suspects in detention are said to have rented the car.

“The blast on the second day of the Muslim holy month of Ramadan hit the Vezneciler district, between the headquarters of the local municipality and the campus of Istanbul University, not far from the city’s historic heart. It shattered windows in shops and a mosque and scattered debris over nearby streets,” Hurriyet said.

The report adds that gunshots were heard after the explosion, but these were apparently warning shots fired by the police, to keep onlookers away from the blast area.

“No group has claimed it, but President Recep Tayyip Erdogan pointed to the involvement of Kurdish militants,” reports the BBC. The Turkish government has been fighting separatists from the banned Kurdistan Workers’ Party (PKK).

Another possible culprit would be ISIS, which has attacked Turkey in the past, and is enraged by Turkey allowing U.S. warplanes to operate out of Incirlik airbase to strike the Islamic State.

“Let me be clear, terrorist organizations distinguishing between civilians, soldiers and police does not mean anything to us. The end target is always human beings,” said Erdogan, as quoted by CNN. “The mission of our soldiers, our police and our city guards is to protect our lives and our property. It is unacceptable that these people are targeted. We will continue our fight against terrorism fearlessly.”

CNN notes that the U.S. Embassy in Turkey called the attack “barbarous” and “senseless,” and promised that the U.S. would “continue to stand shoulder-to-shoulder with Turkey in the fight against terrorism.”

French President Francois Hollande also condemned the attack, saying “this intolerable act of violence must, more than ever, confirm our mutual determination to fight all forms of terrorism.”

CNN notes that a string of terrorist attacks has cut into Turkey’s important tourist industry, a warning echoed by Hurriyet Daily News in reporting that the embassies of the U.S., U.K., and Germany have advised citizens visiting Turkey to exercise greater caution.

“The terror threat has fueled a massive drop in travel to Turkey, previously the world’s sixth-most popular holiday destination. Foreign tourist arrivals fell for a record ninth month in April, declining 28 percent year-on-year, which was the biggest drop on record, according to data spanning a decade,” observes Bloomberg News.