A powerful car bomb exploded Thursday at a university in the northern Spanish city of Pamplona, wounding 17 people and setting a building on fire in an attack blamed on Basque separatists.

There was no claim of responsibility, but officials pointed the finger at the militant Basque group ETA. They said the blast could have caused massive bloodshed because it went off without warning at a busy area of the campus.

"ETA has once again displayed its vileness," said Jose Antonio Alonso, spokesman in Parliament for Prime Minister Jose Luis Rodriguez Zapaptero's Socialist party.

The bomb went off in a parking lot at the University of Navarra, shattering windows and setting vehicles on fire, said Amaya Zaratiegui, spokeswoman for the university's clinic. Navarra is the region of which Pamplona is the capital.

Aparicio Caicedo, a 29-year-old Ecuadorean doctoral student, said he was studying in a library when the bomb went off.

"Suddenly the whole building shook and there was a huge column of smoke. It was tremendous, a huge explosion," Caicedo told The Associated Press by phone.

Television footage showed an unidentified building on fire at the ground-floor level and spewing thick black smoke.

Seventeen people were slightly injured, some by flying glass, Interior Minister Alfredo Perez Rubalcaba said.

He said a man claiming to speak for ETA phoned a warning to authorities in the Basque capital, Vitoria, about an hour before the explosion, saying the bomb was packed in a white Peugeot and would go off at "the university campus."

Police thought that meant Vitoria's university, searched there and found nothing, the minister said. Instead, the bomb exploded without warning in Pamplona, 60 miles (100 kilometers) east of Vitoria.

"Clearly, whoever placed the bomb gave a warning and either intentionally did not give all the information or made a mistake," Perez Rubalcaba said.

"The result is that we could have suffered an enormous tragedy today at the University of Navarra," he said.

Spanish police arrested three suspected members of ETA on Tuesday in Pamplona and another in Valencia but Rubalcaba declined to speculate on whether that could be the reason for Pamplona's being targeted.

Pamplona's mayor, Yolanda Barcina, told Cadena Ser radio she was surprised the casualty toll was not higher.

"We got lucky with the weather. It is raining, and there were fewer people than usual outside," the station's Web site quoted her as saying.

ETA has killed more than 800 people since the late 1960s in its battle to create an independent Basque homeland straddling northern Spain and southwest France.

Navarra borders on the Basque region and is home to many Basque-speakers. ETA says it should be part of the independent homeland it wants to create.

ETA called a cease-fire in March 2006 but resumed attacks in December of that year after peace talks with Zapatero's government failed. Since ending the cease-fire the group has been blamed for seven deaths, including three this year. ETA's last fatal attack was a car bombing in late September that killed an army officer in the northern town of Santona.