Police were called to a home in Rotterdam early Monday and found Halman, 24, bleeding from a stab wound, police spokeswoman Patricia Wessels told the AP. Officers were unable to successfully resuscitate him.
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Officers then arrested Halman's brother, who is 22.
USA Today reported that Halman played 35 games for the Mariners and was batting .230. He was the the only player in the majors born in the Netherlands.
"All my friends and family are so proud," Halman told USA Today in June after his call-up to the Mariners.
"Raw talent. That's what made them sign me."