One Iraqi officer was killed and four police members were wounded today in a bomb blast in the district of Baaj , west of Mosul in northern Iraq.   Police said a police officer was killed and four policemen were wounded in a bomb explosion targeting patrol of border guards west of Mosul.  In a related development, two police officers were seriously injured in an armed attack of a checkpoint west of the city of Fallujah, western Iraq.   Police said armed elements riding a motorcycle opened fire at a checkpoint west of Fallujah, noting that the attack resulted in the injury of two policemen.