How did the waitress die?

Yesterday, Mary, a waitress at Case Hotel, was killed.

Mary was killed in her apartment. When the police arrived at the scene, they found that she was hit in the back of the head with a blunt object. She was lying prone in the middle of the room with a pearl necklace in her hand. Friends all admit that Mary is crazy about money. It is said that she often lends money to her colleagues and then charges high interest. For those who can't pay back the money on time, they are required to use ornaments, dresses and so on as collateral, so everyone hates her. The necklace she was carrying when she died was also taken from her colleague Judy who borrowed money from her.

Strangely, the window was locked and the door was locked from the inside, which means that Mary was killed in the secret room. In this way, Judy, the owner of the necklace, became a murder suspect. However, who knows how Judy entered the room and killed Mary? The criminal hit Mary on the head with a hammer through the door chain. There is a lock on the window and a chain on the door. Even so, it can't be said to be a secret room. Because, if the door with a door chain is unlocked, it can still open a gap that can accommodate a hammer, and criminals use this gap to file a case. Judy deliberately put the necklace where she could see through the door chain, which was a little far from the door. When Mary bent down to pick it up, Judy hit her on the back of the head with a hammer hidden on her outside the door. Because the power of wielding a weapon is limited by space, this blow is not fatal. Mary screamed, grabbed the necklace and ran back to the room. In the end, she died after running a few steps because of her injuries.