CHICAGO — Mandolin player and composer Chris Thile learned the hard way that when you get a call from the 312 area code this time of year, you should probably answer the phone.

Thile is among 23 recipients of this year's MacArthur Foundation "genius grants," which are given in a secrecy-shrouded process. Winners have no idea they've been nominated for the $500,000 awards until they get the call, and nominators must remain anonymous.

Thile ignored the incessant phone calls from the foundation at first, thinking they were election-year robocalls. Then he received an ominous message: "Don't tell anyone about this call."

His tour manager searched for the number online and told him, "It appears to be from something called the MacArthur Foundation." It was a name Thile recognized.

"I think I must have turned white," he said. "I've never felt so internally warm. My heart was racing. All of a sudden, I felt very askew physically. I was trying to catch my breath. ... I thought, 'Oh my God, did I win a MacArthur?'"

The grants, paid over five years, give recipients freedom to pursue a creative vision. Winners, who work in fields ranging from medicine and science to the arts and journalism, don't have to report how they spend the money.

Northwestern University historian Dylan C. Penningroth said he now can expand his search for court records of property owned by slaves in the pre-Civil War South.

"This grant will make it possible for me to think big, to be more ambitious about the time period I cover and the questions I'm trying to answer, like, what's the connection to the modern civil rights era?" Penningroth said.