Fernando Reinares said the militants most likely to have carried out such suicide attacks were seven men who blew themselves up on April 3 as special forces moved in to arrest them in an apartment in the town of Leganes outside Madrid.
"According to data collected so far, it can be deduced that those terrorists were probably planning suicide attacks in the months or weeks after" the March 11 attack that killed 191 people and wounded more than 1,500, Reinares said in an interview.
The data suggests "their terrorist campaign was not going to end on March 11, but was going to go on and include suicide attacks at a later stage," Reinares said.
The seven who died in Leganes included several suspected ringleaders of the train bombing, which was claimed in videotapes by militants who said they acted on behalf of al Qaeda in revenge for Spain's troop presence in Iraq.
His comments came a day after U.S. and Spanish authorities confirmed that a crude sketch of Grand Central Terminal was found at the home of a suspect in the Madrid train bombings, but said it was not considered cause for alarm.
The sketch was "a very basic schematic," and was never deemed cause for alarm, New York City Police Commissioner Ray Kelly said Wednesday in response to a report in the Spanish newspaper El Mundo.
The drawing and other data were on a computer disk seized about two weeks after the train bombings in Madrid that killed 191 people on March 11, 2004, El Mundo reported. Spanish police turned the disk over to U.S. agents from the FBI and CIA in December.
Kelly said the data also was shared with the New York Police Department's counterterrorism division and city transit officials, who concluded the sketch depicted Grand Central.
The material also included photographs, and a drawing of a private building in the city, which Kelly refused to identify. But an analysis found no indication of a terrorism plot, and authorities quickly decided there was no need to alert the public, he said.
Reinares' statements also provide a chilling what-if element to Spain's trauma over the train attacks nearly a year ago. The revelation is not known to have been carried in mainstream Spanish media. Reinares said it was featured in a book by a Spanish investigative reporter that came out two weeks ago.
Western Europe had never suffered a suicide attack. Spanish officials say the militants who killed themselves in Leganes wanted to become martyrs for Islam rather than kill a large number of people. One special forces officer died in the blast.
A string of suicide attacks in November 2003 in Istanbul killed 62 people and were blamed on al Qaeda. The targets were two synagogues, a British bank and the British Consulate.
In the interview, Reinares said the plans for suicide attacks showed that the Madrid train bombers were probably not interested in bringing down the conservative government then in power, which had supported the U.S.-led Iraq war, but rather wanted to go on causing bloodshed.
Former Prime Minister Jose Maria Aznar and his party have insisted the bombing — three days before a general election — was tantamount to a surgical strike against his government, which had defied public opinion by supporting the war and sending 1,300 peacekeepers after major fighting stopped. Socialists who opposed the war won the election and brought the soldiers home shortly after taking power in April.
A total of 74 people have been arrested over the Madrid attack, of which 22 have been jailed on charges of mass murder or belonging to terrorist group. The rest are no longer in custody but are still considered suspects.