Google announced its plan to enter the multi-billion-pound travel market with the buyout of the search specialist last summer.
The proposed $700m (£461m) deal would be Google’s sixth-largest.
It’s understood the deal, which would put Google in direct competition with travel sites like Expedia and Cheapflights, could be blocked depending on the outcome of the investigation.
However, reports suggest the US Department of Justice has increased its requests for information from companies likely to be affected by the transaction.
The search giant faced similar difficulties when it proposed a takeover of mobile ad network AdMob last year.
The US government scrutinised Google’s $750m bid for AdMob on anti-trust grounds.
The Federal Trade Commission built a case against the deal before finally giving it the go-ahead.