Billionaire hedge fund manager Bill Ackman told investors Monday he would return $4 billion raised by the SPAC he launched in 2020.

In a shareholder letter, Ackman said investors in Pershing Square Tontine (PSTH), the blank check firm Ackman created two years ago during a boom in the space, would be redeemed after failing to find a deal that "both meets our investment criteria and is executable."

The Ontario Teachers Pension plan, Canada's largest single-profession retirement system, and investment management firm T. Rowe Price Group are among prominent stakeholders poised to get their cash back.

"We launched PSTH in the depths of the pandemic because we believed that the capital markets would likely be impaired from the economic uncertainty created by the pandemic," Ackman said in the memo Monday.

"The rapid recovery of the capital markets and our economy were good for America but unfortunate for PSTH, as it made the conventional IPO market a strong competitor and a preferred alternative for high-quality businesses seeking to go public."

SPACs — or special purpose acquisition companies — are shell companies that raise money from investors through an initial public offering and then seek targets to bring public.

The blow to Pershing Square Tontine comes amid a sharp slowdown in the SPAC market that has seen several notable deals sputter as investors rotate out of speculative assets amid rising interest rates and uncertainty about the economy.

The risk-off mood has placed a strain on the pipeline for mergers, with several SPACS mergers, including deals for financial media giant Forbes and online ticketing platform SeatGeek, ultimately foregone.

At the start of 2021, SPAC IPO issuance shattered records, with roughly 300 blank-check companies raising nearly $88 billion in the first quarter, data tracked by Pitchbook showed. But only 78 SPAC IPOs took place in the same three-month period this year, with the median deal valued at just half of what it was the year prior, per the latest available figures.

“High quality and profitable durable growth companies can generally postpone their timing to go public until market conditions are more favorable, which limited the universe of high-quality possible deals for PSTH, particularly during the last 12 months,” Ackman said. “While there were transactions that were potentially actionable for PSTH during the past year, none of them met our investment criteria.”

Last year, an attempt by Ackman’s SPAC to buy 10% of Universal Music Group (UMG) – the record label for names including Taylor Swift, Drake, and and Adele – from French media giant Vivendi was nixed by the SEC as the regulator ramped up its scrutiny of blank-check deals.