Three people were wounded, including a female fan, during the incident.
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Washington D.C. police released a statement saying:
"While MPD's investigation is ongoing, it appears the incident involved a dispute between individuals in two vehicles," the joint statement said. "MPD does not believe the Washington Nationals, the ballpark or fans were the target."
Also on Sunday, police offered a $10,000 reward for an arrest and conviction of the shooters and released photos of a gray Toyota Corolla linked to the incident. The stadium did not appear to be the target.
3 wounded in a shooting outside Nationals Park, fans scramble
Former Philadelphia Police Deputy Commissioner Joe Sullivan weighed in on how police and security here at the stadium complex in South Philadelphia would respond, specifically at Citizens Bank Park where the Phillies play.
"You have ... a police headquarters in the stadium that is working with stadium security. In a situation like this, police and stadium security are going to move very quickly to determine whether the gunshots are emanating from the inside or the outside," said Sullivan.
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"If the gunshots are outside it's very important you keep people inside the stadium where it's safe and you know everyone has been screened," said Sullivan noting how every fans must be screened by metal detectors.
He added law enforcement will always respond from a worst-case scenario: "We're going to make an assumption it's an active shooter because we always start at the highest level and work your way down."
Sullivan added preparation on the front end is always what makes a difference to a response.
SEE ALSO: Video shows shooting involving 1-year-old child inside West Philly store
Video shows shooting involving 1-year-old girl inside West Philly store
He said the stadium's law enforcement has numerous protocols and details in place.
"You have police officers that are assigned outside the stadium, you have detailed police officers inside the stadium," he said. "I would say security here is excellent. I wouldn't hesitate to go to a Phillies game."
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The Wells Fargo Center, home of the 76ers and Flyers, along with the Eagles issued statements to Action News. The Phillies and the Philadelphia Union - who play in Chester, Pa. - did not want to comment.
Statement from a Wells Fargo Center spokesperson:
"The Wells Fargo Center has security response plans in place for a wide range of possible situations, including active shooter scenarios. We also have several measures in place, including metal detectors and highly-trained security staff, to prevent firearms from entering the arena. For safety reasons, we do not publicly share our security plans and protocols, but we work hard every day with our security team and with the Philadelphia Police Department to safeguard anyone who visits Wells Fargo Center."
Statement from a Philadelphia Eagles spokesperson:
"The health and safety of our fans and guests is of the utmost importance to the Eagles organization. We have numerous protocols in place that are regularly reviewed and updated in order to provide a safe, secure and enjoyable environment at our venues."