Citing insufficient evidence, prosecutors on Saturday dropped charges against two men arrested on suspicion of helping a gangster assassinate Nagasaki Mayor Itcho Ito last month.

Hiromi Ogawa, 60, a construction company president in the city of Nagasaki, and Masaki Yamashita, 29, a driver for the gangster, were released.

They denied helping Tetsuya Shiroo, 59, kill Ito, saying they were not aware of his plans, according to investigative sources.

Shiroo reportedly confirmed this, saying "I had not informed the two of the plan to kill the mayor," according to the sources.

Investigators had suspected Ogawa, complying with a request by Shiroo, drove him to the site where he shot Ito on April 17, while Yamashita kept a watch on the office of Ito's support group, about 700 meters from the site of the shooting.

Earlier in the month, the Nagasaki District Public Prosecutor's Office indicted Shiroo for shooting and killing Ito and for obstructing election campaigning by killing a candidate.