There was no obvious weapon found at the crime scene, which suggests the killer may have taken it with them and discarded it. Where else was evidence found (and potentially discarded)?

Nathaniel Martinez found evidence in the trash bin at Highland Park on September 14th, 2019. We can link this trash to Summer's murder because some of these items had blood and/or Summer's DNA on them (see the forensic analysis report).

Of the items Nathaniel Martinez found (i.e., those analyzed in the forensic analysis report), which can inflict injuries consistent with those reported in Summer's autopsy report?

Nathaniel Martinez found the following items inside the plastic bag: a trophy, a pocket knife, a hammer, a rope, paper towels, a plastic bag, and gloves. Summer died from blunt force injury to the head. The only items that could likely inflict fatal blunt force trauma are the hammer and the trophy.

Based on DNA findings from the forensic analysis report, which item (trophy or hammer) caused Summer's blunt force injuries?

The trophy had Summer's DNA on it, but the hammer didn't (see the forensic analysis report.) The murder weapon was a trophy (the answer is "trophy").