Read the Rabies Booster Shot Email carefully.

Read the Staff Vaccination Policy Memo carefully.

The Rabies Booster Shot Email says that staff that don't receive the rabies booster shot on September 14th won't be able to access specific animal enclosures or their control rooms until they receive the shot. The Staff Vaccination Policy Memo says these specific animals include large carnivores—like tigers. Calvin's killer entered the tiger control room on September 15th, which means they were up to date with their rabies vaccinations and received the booster shot on September 14th.

Examine the receipt and all physical evidence from Part 1 carefully.

The items on the receipt match two pieces of physical evidence—the confetti and the animal banner fragment—that were found in the Riverside zoo staff room trash.

Examine the "Hey everyone" note carefully.

Mason can be eliminated because he lacked access to a room the killer entered—the tiger control room—on September 15th.

The "Hey everyone" note shows Mason was going to pull together something last minute for Hannah because she was dumped by Calvin the day before her birthday. The Rabies Booster Shot Email shows Hannah's birthday was on September 14th, which is the same day as the Rabies Booster shot and the same day someone purchased birthday/party supplies at Clover Variety (see the Receipt). We can infer Mason purchased the supplies—it's consistent with the 'Hey everyone" note, as well as the birthday supplies found in the staff trash room (which match the items listed on the Clover Variety receipt).

The receipt shows Mason purchased items for Hannah's birthday at 11:03AM on September 14th, 2023, which means he missed the rabies booster shot (it was given at 11AM that day) and had his access to the tiger control room revoked. Mason couldn't have killed Calvin because he couldn't enter the tiger control room on the 15th like Calvin's killer did (see the Crime Scene Report).