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By Scott Malone

WATERTOWN, Mass., April 19 (Reuters) - Police believe they have shot dead one of the suspects in the Boston Marathon bombing and were searching for another who was at large in the Boston suburb of Watertown, officials said on Friday.

"One suspect dead. One at large. Armed and dangerous. White hat suspect at large," Boston Police Commissioner Ed Davis tweeted.

One of the bombing suspects was photographed wearing a white hat.

Later, Davis told reporters of the man at large: "We believe this to be a terrorist. We believe this to be a man who has come here to kill people. We need to get him in custody."

The massive police operation was under way after the U.S. Federal Bureau of Investigation (FBI) on Thursday released pictures and video of two suspects in the Boston Marathon bombing, enlisting the public's help in identifying two men wearing backpacks and baseball caps in the crowd minutes before bombs exploded near the finish line.

A massive police operation was under way in the Boston suburb Watertown, where Massachusetts State Police warned people not to open their doors.

Police were conducting a door-to-door, street-by-street search due to what it called a fluid situation. (Additional reporting by Alex Dobuzinskis in Los Angeles and David Bailey in Minnesota; Writing by Daniel Trotta; Editing by Mary Milliken and Lisa Shumaker)