Carlos Urrutia, the Colombian ambassador to the U.S., resigned Tuesday amid mounting pressure over a series of controversial land sales facilitated by his former law firm.

In a letter to President Juan Manuel Santos, Mr. Urrutia said that he didn't want to be seen as using his position of ambassador amid rising questions about his role in the transactions, which involved large corporations buying up thousands of small parcels awarded to individual, low-income farmers through a federal program.

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