Washington (CNN) -- A Pentagon official said he believes the shooting early Tuesday at the U.S. Defense Department headquarters was a "random incident." "We are looking at all the possibilities," Steven E. Calvery, director of the Pentagon Force Protection Agency, said at a news conference late Tuesday morning. "What we have is an isolated incident, so far." Pentagon police officers, as well as several construction workers in the area, heard at least five shots fired around 4:50 a.m., Pentagon officials said. As of midday Tuesday, authorities had discovered two bullet fragments in third- and fourth-floor windows on the south side of the building, said Calvery. That part of the Pentagon was empty at the time of the shooting, as it is in the process of being renovated. The bullets shattered but did not go through the windows, which are bulletproof, according to Calvery.
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NEW: An official sees no apparent link between Pentagon, Marine Museum shootings
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Several shots were fired early Tuesday into an unoccupied part of the Pentagon
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A top Pentagon security official is calling the shooting "an isolated incident, so far"
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Two bullet fragments have been discovered, and the investigation is ongoing
Asked if there were any ties between that and the @placeholder shootings, a law enforcement official said that "except for the similarity in the incidents -- windows shot out at military facilities -- there is nothing to connect these incidents at this point."