3 Cases of Detective Stevens

Murder Out of Friendship
Mary and Ben lived in the same house with four of their friends. One day, when Ben came home from work, he found Mary dead on the couch and immediately called the police.
A detective arrived at the scene and realized that Mary had been killed very recently. He questioned all four friends:
Mia was in the kitchen and said she had come home from work two hours ago and had gone straight to the kitchen.
John was sitting on a lounge chair in the garden reading a book and, according to him, hadn’t left that spot since morning.
Jennifer had been swimming in the pool and said she’d spent at least three hours there.
Jane was in her room drawing.
The detective asked everyone to show their hands and immediately realized that one of them was lying, which meant that person was the murderer.
How did Stevens identify the murderer?

The Murder of an Aristocrat
George Smith was murdered on Sunday evening. At that moment, there were five people in the house: Mr. Smith’s wife, the personal chef, the butler, the maid, and the gardener. They all told Detective Stevens what they had been doing that evening:
Mrs. Smith was sitting by the fireplace reading a book.
The chef was preparing breakfast.
The butler was supervising the workers in the living room.
The maid was washing the dishes.
The gardener was watering the plants in the greenhouse.
Immediately after the conversation, the detective arrested the murderer.
Who is the murderer, and how did Stevens figure out who the culprit was?

Blood Ties
The millionaire was killed in his home by a gunshot to the forehead. The detective questioned the millionaire’s three sons—Jack, John, and James—who had been at home.
Jack said that that evening, he, John, and their father had been at a party. They arrived home and entered the living room: their father went in first, followed by John, and then Jack himself. When their father reached the stairs leading to the second floor, James entered the house and shot their father.
John confirmed Jack’s account.
James said that he had been visiting friends that evening, and when he entered the living room, his father was already dead.
The detective immediately realized who had killed the millionaire.
Who was it, and how did Stevens figure it out?