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Maybe they thought it was not serious or anything.

Incompetence.

IIRC they wrote up a search warrant, but it ended up just sitting in a desk somewhere.

Might as well ask why nobody decided to intervene when Nikolas Cruz was throwing around statements about wanting to shoot up schools and kill people. Someone was asleep at the wheel, so to speak.

They wrote Eric's threats off as teenage bullshit.

Plus, the boys weren't taken seriously in general. Even at their job they would talk about blowing up Columbine and killing the jocks and their boss didn't even warn the cops that they might have a few screws loose.