Jeffrey Gundlach, the widely followed investor who oversees DoubleLine Capital, said on Thursday the biggest surprise from the U.S. Federal Reserve's decision to keep rates unchanged was policymakers' focus on recent global economic and financial developments.

"Basically what they said is we didn't tighten because of global markets. This is a big deal," Gundlach said. "The global growth isn't there and they are really scared to reverse course if they raise rates."

The Fed on Thursday kept interest rates unchanged in a bow to worries about the global economy, financial market volatility and sluggish inflation at home, but left open the possibility of a modest policy tightening later this year.

Gundlach, whose Los Angeles-based DoubleLine Capital had $76 billion in assets under management as of June 30, has maintained since May the Fed will not raise rates at all this year and added at the conclusion of the Fed's meeting on Thursday: "There is not enough global growth to go around and the Fed realizes it. Global growth has to improve for the Fed to raise rates."

Going into this week's Fed decision, Gundlach told Reuters on Wednesday that U.S. Treasuries, and particularly the five-year Treasury note, looked attractive because their yields have moved higher in recent weeks.

U.S. five-year Treasury yields hit over a one-week low of 1.48 percent during Fed Chair Janet Yellen's press conference on Thursday.

U.S. two-year note yields dropped about 13 basis points to mark the biggest daily decline in 6-1/2 years on Thursday. Benchmark 10-year Treasury note yields also plunged, trading around 2.20 percent from 2.3 percent on Wednesday, the highest in nearly seven weeks.