He was being looked after by family friends while his parents were out.
Michelle Booker-Hicks shot Rickey Wright, 36, in the head after he tried to steal her car from a gas station.
Monalisa Perez, 20, fatally shot her boyfriend Pedro Ruiz during a botched YouTube stunt.
Authorities have identified one other suspect and said others could be involved.
The gun fell out of the agent's holster as he was dancing.
State authorities praised the off-duty officer's "technically perfect" response.
The pro-Trump sign in Maryland has been the subject of numerous complaints from concerned residents.
"There's nothing else I think about wanting to do," said Fred Guttenberg. "There's nothing else I care about."
Father-of-two Devyn Holmes, 26, is now in critical condition and on life support at Ben Taub Hospital.
According to reports, the accident marks the 150th time a child has been injured or killed in a shooting in 2018.
"We now have students becoming the mass shooting generation, and that is wrong," the mayor of Seattle said.
Even Bill Murray is inspired by those young activists.
The number of children killed by gunfire in the United States since the 2012 mass shooting at Sandy Hook Elementary School is greater than the total of U.S. soldiers killed in overseas combat since 9/11, according to a Department of Defense report.
That tragedy marked the 123rd child under the age of 11 killed or wounded in gunfire since January 1. It was also the second fatal shooting of a child shooting of that day.
Police named the victim as 39-year-old Dannta Holmes.
Christopher Morton was fatally shot, while Nathan Bettencourt and Nicholas Kasper also suffered gunshot injuries when they arrived at the wrong house.
New polls show rising support for gun safety measures in the wake of recent mass shootings.
“Thoughts and prayers are not enough. We have to help solve the problem that’s in front of us.”
The school shooting suspect accused of killing 17 people stopped his assault when his gun jammed, a report says.