General Discussion

About two hours before the attack, the mother of one of the suspects had called police to report that her son had left home with several of her guns and her car.
...
A note the youth left behind also included "generalised hate rhetoric and hate speech", he said.

Wahl added that the note contained no specific threat to the mosque, or to any other location or individual.

Investigators went to a local high school, where one of the teens was a student, as well as a shopping mall where the car had been tracked.

When the shooting took place, officers were still speaking to the mother and were only a few blocks away from the mosque.

https://www.bbc.co.uk/news/articles/c5y7q66gq56o