Brain puzzle

A challenge
We saw the following brain puzzle in the June 2013 issue of Engineering & Technology. Can you work the answer out? As a bonus, can you work it out using the statements from less than all four people?

 

The puzzle: who stole the coins?
When your back was turned, someone stole all the coins.

Apart from yourself, there were four people in the room, any one of whom could have taken the coins.

They agree to let you find out who did it by logic. Each of them must make three simple statements, two of which must be true and one of which must be false.

John says “I didn’t take the coins. I have enough cash. Mary stole them.”

Kevin says “I didn’t take the coins. I have never stolen anything in my life. Mary stole them.”

Liz says “I didn’t take the coins. I know who did take them. John has plenty of cash.”

Mary says “I didn’t take the coins. Kevin took them. John is innocent.”

Assuming it is true that each made two true statements and one false statement, who took the coins?

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