A bomb targeting worshippers attending Friday prayers at a Sunni mosque north of Baghdad killed 16 people and wounded dozens more, the top medical official in the province said. It was the second major attack in as many days in Iraq and highlighted the difficulties Iraqi security forces still face in protecting their own people as American forces prepare to leave by the end of the year. Sixteen dead were brought to the main hospital in Tikrit, said the medical official, Dr. Raeid Ibrahim. He said an additional 54 people were wounded, indicating that the death toll could rise. Tikrit is the home town of deposed ruler Saddam Hussein, and many of his relatives and associates live there. There was no immediate claim of responsibility for the bombing.