Virtu Financial Inc., the high-frequency trader that announced plans last month to sell shares, has delayed the deal, two people with knowledge of the matter said.

Virtu’s bankers won’t start marketing the initial public offering until after April 20, delaying the process from this week, according to the people, who asked not to be named because the decision is private.

The delay comes amid unprecedented scrutiny of high-frequency traders. “Flash Boys,” the Michael Lewis book released Monday, argues that high-speed traders, Wall Street brokerages and exchanges have rigged the $23 trillion U.S. stock market. New York Attorney General Eric Schneiderman is examining privileges such as enhanced data feeds marketed to high-speed firms, while the Federal Bureau of Investigation is looking into whether those traders are breaking U.S. laws by acting on nonpublic information.

Chris Concannon, the president of New York-based Virtu, declined to comment on the IPO. The trading firm provides quotes in more than 10,000 securities and contracts on more than 210 venues in 30 countries, according to its IPO filing.

In its IPO filing released in March, Virtu said U.S. derivatives regulators are looking into its trading practices.

The U.S. Commodity Futures Trading Commission has asked about Virtu’s “participation in certain incentive programs offered by exchanges or venues” from July 2011 to November 2013, Virtu said in its IPO document. Virtu said it doesn’t believe it broke the law, “but we cannot predict the outcome of the inquiry.”