An official warned that "an immense tragedy" appeared to be unfolding.
The same location was the scene of a March 2017 terror attack that killed five people.
Two other children were being trained to attack schools, law enforcement and banks, prosecutors allege.
"I am staying in the race and intend to win and lead by example from now on," Melissa Howard said.
"I am a German when we win, but I am an immigrant when we lose," said Mesut Özil, who recently quit the national soccer team, citing racism.