political movie about atempted assasination

About 15-20 years ago, on TV,I watched a movie in which an inspector was in charge of an attempted assassination against president. He collected a series of pictures after the attempt, and tried to contact them to ask what they had seen. However, he found out they were all mysteriously dead. I also remember the last scene in which he was calling an authority to give report, when a bullet came through right from his large window and killed him.

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