Story Highlights• Powerful car bomb explodes in parking lot at Spain's busiest airport
• Spain says bombing is a violation of nine-month cease-fire with separatist ETA
• Blast followed warning to police, claimed by Basque separatists ETA
• One man reported missing after explosion
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MADRID, Spain (CNN) -- Spain has blamed a powerful bomb explosion Saturday at the country's busiest airport on Basque separatist group ETA, declaring it a violation of a nine-month cease-fire.
The bomb at Barajas International Airport is "absolutely incompatible with the permanent cease-fire that ETA declared nine months ago," Spanish Prime Minister Jose Luis Rodriguez Zapatero said during a nationally televised news conference.
He ordered the government to halt any peace efforts, but he declined to say that the attack "breaks the cease-fire," which was the way Interior Minister Alfredo Perez Rubalcaba described it to reporters a few hours earlier.
A police source had initially said that the blast, which occurred in a stolen van and damaged a parking lot and nearby Terminal 4, caused no serious injuries. (Watch smoke billowing from airport )
But Rubalcaba said that may not be the case: Police were looking for two people reported missing after the blast -- one of them a 19-year-old Ecuadorian national who had stayed in his car as a companion went into the airport to pick up someone. The other person was not described.
If it turns out that either person is dead, the death would be the first attributed to ETA in three years.
Beginning about an hour before the 9 a.m. (3 a.m. ET) explosion, three calls to authorities warned of the event, the third acknowledging it as an ETA blast, Rubalcaba said.
After several hours, regular air traffic at Terminal 4 resumed Saturday. Terminal 4 opened this year and primarily serves Iberia Airlines, a private company.
Flights at the airport's other three terminals were not interrupted.
Before Rubalcaba's remarks, opposition leader Mariano Rajoy of the Conservative Popular Party told reporters, "This confirms what we knew: that ETA is a criminal organization that does not want any peace."
Zapatero was out of Madrid at the time of the blast, but he hurriedly returned to address the nation at 6 p.m. (noon ET).
Samantha Graham, an employee of CNN's parent company Turner Broadcasting, was in the terminal and heard the blast. Graham described a huge column of black smoke billowing into the sky.
She said hundreds of people evacuated the terminal through jetways and gathered outside on the airport tarmacs.
The Madrid airport is Spain's busiest, handling more than 40 million passengers a year, according to the airport's official Web site.
ETA announced a "permanent" cease-fire last March, raising hopes for an end to nearly 40 years of separatist violence blamed for more than 800 deaths and thousands of other casualties.
Last fall, low-level street violence resumed in the northern Basque region -- which ETA is seeking to make an independent nation -- as pro-ETA youths burned buses and automatic bank teller machines and carried out other attacks.
In late October, authorities blamed ETA for the theft in Nimes, southern France, of 350 handguns and 10,000 rounds of ammunition from an arms factory.
Arnaldo Otegi, a leader of the outlawed Batasuna party, which authorities link to ETA, on Saturday blamed the government for the roadblocks in the peace process. But he told reporters there's still a chance for peace, even after the bomb.
CNN's Madrid Bureau chief Al Goodman contributed to this report.
A huge column of smoke billows from Barajas airport.