The game was rescheduled as part of a day-night doubleheader on Tuesday.
"It's a very emotional day. An extremely horrific act happened very near to the ballpark," Nationals general manager Mike Rizzo said. "Our thoughts and prayers go out to all the victims over there and all the people affected by this. We felt it was inappropriate to play a major league baseball game with such tragedy right down the street."
Located just blocks from the Navy Yard, one of the stadium's parking lots was used for families of the Navy Yard employees to meet following the evacuation of the facility.
"Major League Baseball mourns those who have been lost to this senseless tragedy," Commissioner Bud Selig said. "Major League Baseball's security department will continue to track this matter in consultation with the Nationals and under the guidance of the local authorities."
The postponement was announced shortly after 3 p.m., about four hours before scheduled start.
"We have to be in contact with the federal authorities, the state and D.C. authorities to have a coordinated effort, and then whenever you cancel a game, MLB is involved and we have to go through the correct procedures for that," Rizzo said.
With some streets north of the stadium closed, there was heavy traffic on South Capitol Street, parallel to the third-base. Potomac Street, parallel to first-base line, was blocked by police. Police activity was visible around the stadium, and sirens could be heard throughout the afternoon.
Players from both teams were in the clubhouses at the time of the postponement along with stadium employees.
The doubleheader will open a three-game series. The Braves have a 10-game lead over second-place Washington in the NL East, and the Nationals began Monday 4½ games behind Cincinnati for the final NL wild-card berth.
Washington starter Dan Haren and Atlanta's Mike Minor were pushed back to Tuesday afternoon, and the Natiinals' Tanner Roark opposes Freddy Garcia in the night game.
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