According to the Financial Times, closure could be delayed past the second quarter of the year, taking it outside the original target set by the social network in its IPO documents.
Two people apparently familiar with the matter told the newspaper that the FTC has been gathering information from competitors Google and Twitter.
Stanford Law School professor Mark Lemley added: "They're going to want to take some months to investigate and understand the market and other players.
"And there may be more parties with an interest in submitting information."
David Balto, an ex-FTC policy director and now an antitrust lawyer, described Facebook's projection that they could close the deal in Q2 as "terrifically optimistic".
Facebook will not be allowed to integrate Instagram staff with mobile app development expertise or any Instagram technology until the deal is approved.