Special purpose acquisition companies (SPACS) have raked in a record $100 billion through initial public offerings so far in 2021, data from Refinitiv showed, in what has been Wall Street’s biggest gold rush in recent years.

About 341 global SPAC IPOs have been brought to the market since the start of this year, with U.S.-listed SPACs dominating the marketplace, according to the data.

“Amid all the uncertainty caused by the pandemic, SPACs became a good option to access the public markets compared to the more difficult and lengthy traditional IPO route,” said Lucille Jones, Deals Intelligence Analyst at Refinitiv.

SPACs are shell companies that raise funds via a listing to acquire a private company with the purpose of taking it public, allowing such targets to sidestep a traditional IPO.

However, as SPACs and shell companies gain popularity, the U.S. Securities and Exchange Commission (SEC) is stepping up scrutiny against them and has opened up an inquiry into Wall Street’s blank check acquisition frenzy.

“The pace of new listings has slowed considerably since its peak in March. Regulators stepped in with investor warnings and a push for more disclosures and transparency around SPACs, which dampened the mood and caused activity to drop significantly in April to the lowest monthly total since June 2020,” Jones added.

Last month, the regulator issued accounting guidance for SPACs, which called into question whether warrants issued by hundreds of SPACs could be considered equity instruments, while also suggesting that many SPACs may potentially have to refile their financial statements to account for the warrants as a liability.