Simplify the following text for a Grade 11 native English-speaking person.
Original: According to news reports , various agencies demanded new tires , and Firestone conducted an investigation of the complaints , tested the tires and asserted that the tires had been abused or under-inflated .
Simplified: Firestone conducted an investigation of the complaints , tested the tires and asserted that the tires had been abused or under-inflated .