Spies Like Us

In the ancient kingdom of Marigold, there were three types of people: knights, knaves and spies. Knights always told the truth, knaves always lied, and spies could tell the truth or lie as they pleased. The vast majority were either knights or knaves, only very few people were spies. The perceived randomness and uselessness of a spy’s statements was quite frustrating to most people, so much so that in fact being a spy was made illegal. It had been decreed that all spies should be imprisoned.

This puzzle is about a trial in court, with five suspects: Alec, Bob, Carl, David, and Eric. It was known that exactly one of these five citizens was a spy. Also, among the five suspects, they all knew the identities of each other. To find out who the spy was, they were each asked in turn to make two statements.

Alec spoke first: “David is a knave. If Eric is the spy, then Carl is a knight.”
Bob spoke second: “If Alec is the spy, then Eric is a knave. I am the same type of person as David is.”
Carl spoke third: “David is the only knight among us. Alec is the spy.”
David spoke fourth: “If Carl is the spy, then Eric is a knave. I am not the spy.”
Eric spoke fifth: “The spy has already lied twice. I am not the spy but neither is Alec.”

The spy was convicted and sent to jail. But can you tell me who the spy was?