As speculation of an imminent Federal Reserve rate hike mounts, the largest treasury bond-related exchange traded fund is bleeding assets at its fastest pace ever.

The iShares 20+ Year Treasury Bond ETF (NYSEArca: TLT) experienced its worst three-week outflow since it began trading in 2002 as investors yanked $1.5 billion from the fund, with $633 million pulled last week, reports Alexandra Scaggs for Bloomberg. [Rising Inflation Expectations Weigh on Treasury Bond ETFs]

The outflows suggest growing concern that the Fed is readying to raise rates. Investors have already been dumping Treasuries, with the benchmark 10-year Treasuries jumping from the 1.67% low at the start of February and to its current level at about 2.08%.

Despite the large redemptions, TLT has gained almost 1% over the past three weeks as Treasury yields remained relatively flat over the period.

“For a while, the market had been really underestimating the chances of a rate hike,” Matt Whitbread, co-manager for Barings Asset Management, said in the article. “We’ve seen it come around.”

According to futures data, traders are expecting a 52% chance the Fed will hike rates by the end of September, compared to 39% at the end of January. [Alternative Bond ETFs for a Rising Rate Environment]

Long-term bonds are most at risk of a rate hike. For instance, TLT shows a 17.7 year duration – duration is a bond fund’s measure of interest rate sensitivity. Consequently, a 1% rise in rates could translate to about a 17.7% decline in TLT’s price. In contrast, bond funds with shorter durations would have a lower sensitivity to rate changes.

Meanwhile, as investors exited the long-term bond ETF, some traders have piled into inverse Treasury options to hedge portfolios or capitalize on a potential quick profit. For example, the ProShares UltraShort 20+ Year Treasury (NYSEArca: TBT), which seeks to deliver twice the daily inverse performance of the Barclays Capital US Treasury 20+ Year Treasury Bond Index, has attracted $65.7 million in net inflows and TBT’s triple-leveraged cousin, the Direxion Daily 20-Year Treasury Bear 3X ETF (NYSEArca: TMV), has added $25.3 million over the past three weeks, according to ETF.com.

iShares 20+ Year Treasury Bond ETF

For more information on the Treasuries market, visit our Treasury bonds category.

Max Chen contributed to this article.