Belgian police arrested 14 people in a series of raids Wednesday as part of a probe into a woman thought to be the first European female suicide bomber in Iraq, the authorities said.
A man was also arrested in the Paris region in connection with the inquiry.
The woman is suspected of killing up to six people in a suicide attack in the Baghdad region on November 9. She had "a Belgian name", and converted to Islam shortly after marrying a religious extremist, prosecutors said.
Belgian justice authorities were expected to announce on Thursday whether nine of the suspects would be held in custody. They could face charges of "membership of a terrorist organisation", federal prosecutor Daniel Bernard told reporters.
He said the raids "were the result of an accelerated four-month investigation, based on information from police and intelligence services which show the presence in Belgium of ... structures to help people wishing to voluntarily fight in Iraq."
"People coming from Belgium took part in an attack or attacks in Iraq and others were getting ready to go," he said.
Of the 14, nine were Belgians -- two of them of Tunisian origin -- three were Moroccans and two were Tunisians. Documents were seized but no explosives were found.
Belgian media said that the woman travelled to Iraq via Turkey with her husband, who was of Moroccan origin. A police spokesman said the man was "killed by American soldiers" in Iraq.
Police are believed to have been watching her family for some time but apparently speeded up their operation after information about her was revealed on French radio on Tuesday.
The bomber was not previously known to police.
Earlier, after the raids were launched, Leive Pellens, a spokeswoman for the federal prosecutors office, said "This operation is clearly a terrorist operation."
She said that sweeps took place in the capital Brussels, Antwerp to the north, Charleroi in the south and Tongres in the east.
In France, a man was also arrested in the Paris region in connection with the probe, sources close to the case there said. Two other people living with him were detained.
The man is suspected of being "in contact with a cell" helping to send potential fighters or bombers to Iraq, where attacks on US troops and civilians are a daily occurrence almost two years after the war to oust Saddam Hussein.
One source said he had known the woman bomber's husband.
Belgium has served as an unwitting rear-base for terrorist groups in the past.
Thirteen suspected members of a radical Moroccan Islamic group went on trial in Brussels at the beginning of the month on suspicion of links to the Madrid train bombings and a series of attacks in Casablanca.
The 13 men are believed to be members of the Moroccan Islamic Combat Group (GICM). They are accused of "membership of a terrorist group", using forged documents and residing illegally in Belgium, among other charges.
The March 11, 2004 train bombings in Madrid left 191 people dead, and 45 people were killed in the May 2003 Casablanca attacks.
Two days before the September 11, 2001 attacks in the United States, Afghan opposition leader Ahmad Shah Masood was killed in a suicide bombing involving a man with a Belgian passport posing as a journalist.