MEDAN, Indonesia--A 24-year-old university student blew himself up outside police headquarters in the Indonesian city of Medan on Wednesday, wounding six people, just a month after an Islamist militant attacked a former security minister.

National police spokesman Dedi Prasetyo said the student was a "lone wolf" suicide bomber, although authorities were still investigating whether he had links to any militant groups.

Indonesia, the world's most populous Muslim-majority country, has suffered a resurgence in homegrown militancy in recent years, with police frequently the target of attacks.

Prasetyo said four police officers and two civilians, one of whom was a police employee, were wounded by the blast in a car park at Medan police headquarters shortly before 9:00 a.m. (0200 GMT).

"All pieces found at the scene will be tested by a forensic lab to determine the type of bomb," Prasetyo told a news conference, noting that nails, cables, a switch button and the suspect's body parts had been recovered.

Indonesia's anti-terrorism unit, Densus 88, was investigating whether the bomber had any links to a radical group like the Islamic State-inspired Jamaah Ansharut Daulah (JAD), which has carried out a series of attacks in the country, he said.

Television broadcast images showed smoke and a shower of fragments coming from the parking lot area and people rushing out of buildings around the headquarters after the blast.

"Perpetrators and terrorist groups will be chased, captured and brought to justice by our law," Fadjroel Rahman, a spokesman for President Joko Widodo said in a statement, cited by media.

Indonesian airport operator Angkasa Pura II said it had tightened security at 19 airports following the bombing.

The suspected attacker, who was wearing a motorcycle taxi driver jacket and had a back pack, is believed to strapped the bomb to his body, Prasetyo said. Earlier, a provincial police spokesman said the suspect had his bag checked before entering the car park near an area where people were queuing for police verification certificates.

The attack comes a month after a suspected Islamist stabbed and wounded Wiranto, Indonesia's former security minister, after he had opened a university building. Wiranto, who like many Indonesians uses just one name, has since been discharged from hospital after undergoing surgery.

The government scrambled to tighten its anti-terrorism laws after a series of suicide bombings linked to the JAD group killed more than 30 people in the city of Surabaya last year.

Foreseeing an increased threat of attacks from Indonesians who joined Islamic State and have begun returning from the Middle East, police have detained hundreds of suspects since the start of the year.

Stanislaus Riyanta, a terrorism expert, said the attack in Medan could be in retaliation for the death of Islamic State leader Abu Bakr al-Baghdadi, who killed himself last month during a U.S. commando raid on his compound in Syria.