MADRID, Spain (CNN) -- A car bomb exploded Thursday in a parking lot at the University of Navarra in Pamplona, Spain, sparking a massive fire, but "luckily," no one was nearby and there were no deaths, Spain's interior minister said.

"There could have been an enormous tragedy today at the University of Navarra," Interior Minister Alfredo Perez Rubalcaba said hours after the attack in a televised address.

Hospital officials told CNN that 27 people suffered minor injuries, including one American woman and five other non-Spaniards. The Basque separatist group ETA is suspected to be behind the attack.

The car, packed with an unknown quantity of explosives, was parked in a lot near the university's library and detonated shortly after 11 a.m. local time (6 a.m. ET), Rubalcaba said. Watch witness describe the blast »

Campus authorities shut down the entire campus shortly after the blast to search for a possible second explosive device near the science building, about a half kilometer (.3 mile) from the car bombing, according to CNN's partner station in Spain, CNN+. Watch report on Pamplona car bomb »

No device was found and the search was called off Thursday afternoon, according to a university spokesman and a representative for the national government's main office in Navarra. Most of the campus was reopened; only the area surrounding the bomb blast remained cordoned off, they said.

Classes are expected to resume as scheduled on Friday, the university spokesman said.

The car bomb sparked a massive fire in the campus' Central Building, where about 400 students and staff were located at the time, university spokesman Jesus Diaz told CNN+. Video from the scene showed fire engulfing part of the building and thick clouds of black smoke billowing over the campus.

Authorities in Spain's Basque province of Alava received a warning call at 9:50 a.m. in the name of ETA. Watch aftermath of the blast »

The warning call to the DYA emergency services in Alava said the car bomb would strike a university campus in the city of Vitoria about an hour after the phone call, prompting emergency officials to search that campus and determine that the call was merely a false alarm, Rubalcaba said.

"So whoever planted the bomb either gave an intentionally misleading warning call or they made a mistake," he said in his televised address.

According to CNN+, the University of Navarra has been targeted by ETA six times in the past 30 years, and one faculty member said it is unclear why.

"It is not a political university," Professor Maria Teresa La Porte told CNN. "We don't understand."

"If they are looking for innocent victims, the place to find them is here," she said. "There is no way to stop them."

She said it is an open campus with students from various backgrounds -- including Basque -- where people "can come here completely freely."

The blast could be heard across the campus of the private school, which has 12,000 students -- including more than 1,000 scholars from countries outside Spain. It is also home to Spain's top-rated journalism school, which is located not too far from the blast site.

A spokeswoman for Clinica Universitaria de Navarra -- located across the street from the university -- said that 24 patients were treated for injuries ranging from glass cuts to hearing problems. Most have been released and only two are expected to remain overnight, she said.

Two other patients are being treated at Hospital Virgen del Camino, also in Pamplona, but their nationalities were not clear.

Those being treated at Clinica Universitaria de Navarra included 18 Spaniards and six others with the following nationalities: American woman from California, age 20; Italian woman, age 28; Portuguese woman, age 23; Chilean woman, age 25; Peruvian man, age 28 and Philippines man, age 34.

Thursday's explosion came just days after Spanish police arrested four suspected ETA members early Tuesday. Authorities accused them of forming a terrorist cell that was ready to attack, the Spanish Interior Ministry said in a statement.

Three of the suspects were arrested in or near the city of Pamplona, in Spain's northern Navarra region, which has Basque roots and was to be the base for the alleged cell, the statement said.

The fourth suspect was arrested in the city of Valencia, on Spain's eastern coast.

Police seized two revolvers and ammunition; various timers that might be used for bombs; detonating cord; items that might be used to make a bomb attached to the underside of vehicles; various substances that might be used to make explosives; and computer documentation, the ministry said.

All four suspects -- three men and a woman -- were born in Pamplona and range in age from 26 to 29, the ministry said.

ETA is blamed for more than 800 killings in its four-decade-long fight for Basque independence. The European Union and the United States list ETA as a terrorist group.

CNN Madrid Correspondent Al Goodman contributed to this report.

All About Basque Country • Spain • ETA Separatist Group