(CNN) -- Twelve men of Somali origin arrested in the Netherlands last week on suspicion of planning a terrorist attack have been released, but three of them are still being treated as "suspects," a government official said on Thursday.
Eleven were released on Tuesday, and the country's Public Prosecution Service spokesman, Wim de Bruin, told CNN on Thursday that the last has been freed.
"The attack was said to be imminent, so the national police started an investigation which led to the arrest of 12 Somalis later on Friday in Rotterdam," de Bruin said on Saturday. "We are not sure about what the target was, how they were going to carry out the attack or when."
The men, between ages 19 and 48, were arrested in various locations. Police have searched a pawn shop, four homes and two hotel rooms, de Bruin said. Six of the men live in Rotterdam and one is from Denmark. Some are Dutch citizens, but all are of Somali origin, de Bruin said.
No weapons or explosives were found, he said.
War-torn Somalia has a virtually powerless central government and is home to several militant groups, including Al-Shabaab, which controls much of the country.
Al-Shabaab has claimed responsibility for several attacks, including a series of bombs that tore through an Ethiopian restaurant and a rugby center in Kampala, Uganda. Officials said the July 11 blasts in the Ugandan capital, which killed 79 people, probably were set off by suicide bombers.
The United States this year charged several people with trying to funnel money or otherwise assist Somali extremists. Al-Shabaab has been linked to al Qaeda.
Somali President Sheikh Sharif Sheikh Ahmed in September asked other countries to train Somalia's forces to help counter militant groups.