The death toll has risen to 34.
Gunmen stormed the building in the Iraqi city of Tikrit on Tuesday wearing military uniforms and suicide bomb belts. The attackers shot several hostages as Iraqi security forces closed in and engaged them in a gun battle.
Three local lawmakers are reportedly among the dead, and at least 65 people were wounded.
"We've lost contact with three provincial council members who were inside the building when the attack took place," provincial governor Ahmed Abdullah said in a telephone interview from Amman, Jordan, where he was receiving updates on the assault via mobile phone.
He described a fierce shootout between at least eight gunmen, who have overtaken the council headquarters' second floor, and Iraqi security forces who surrounded the building. He said the attackers were hurling grenades at Iraqi forces.
Security officials are blaming al-Qaeda in Iraq for the attack.
The Associated Press contributed to this story.