The biggest exchange-traded fund investing in long-term Treasuries is losing investors at the fastest rate ever on concern the Federal Reserve is moving closer to interest-rate increases.

The iShares 20+ Year Treasury Bond ETF (Symbol: TLT) posted its biggest three-week outflow since its inception in 2002, according to Bloomberg data. The sales came even after Treasuries rallied last week as weaker domestic growth damped the outlook for inflation and combined with the start of additional monetary stimulus in Europe to boost the allure of U.S. government debt.

“For a while, the market had been really underestimating the chances of a rate hike,” said Matt Whitbread, who co-manages about $11 billion across asset classes for Barings Asset Management in Boston. “We’ve seen it come around.”

The outflows indicate a broadening consensus that the Fed is getting ready to raise short-term rates. That’s a turnaround from the beginning of the year, when investors were concerned about deflation because of a drop in oil prices and lackluster growth in Europe. Since the end of January, Treasury yields have increased alongside forecast for a Fed rate move.

Last week, investors pulled $633 million from the $6.2 billion iShares 20+ ETF, which tracks returns of U.S. government bonds that mature in 20 to 30 years. That means investors have withdrawn $1.54 billion during the past three weeks, according to analysis of Bloomberg data.

The ETF has returned 1.9 percent this year after gaining 27 percent in 2014. Treasuries with maturity of 10 years or longer have lost 1.7 percent this month, according to the Bloomberg U.S. Treasury Bond Index 10+ Years.

Fed Watch

Speculation mounted that the Fed will alter or remove the word “patient” from its policy statement after the latest data showed that the U.S. economy created more than 200,000 jobs for 12 straight months. The central bank has held its federal funds target rate a virtually zero since December 2008.

Traders are forecasting a 52 percent chance that the Fed will raise rates by the end of September, according to futures data, up from 39 percent at the end of January.

“The Fed is poised to raise short-term rates,” said Wayne Lin, fund manager at New York-based QS Investors, a Legg Mason affiliate that manages $6 billion. “That’s going to lead to volatility in the Treasury markets.” Because of that, he said recently pulled 5 percent of his funds out of bonds, and put that cash into stocks.