Google's recent purchase of mobile advertising specialist AdMob might come back to haunt both companies: reports state that the US Federal Trade Commission is taking an interest in the deal.

According to BusinessWeek, the FTC is currently asking Google's competitors for sworn declarations in the opening salvos of an investigation into allegations of decreased competition in the mobile advertising marketplace as a result of the deal.

The unnamed industry sources are quoted as claiming that two separate companies - likely to include smartphone rival Apple, which recently bought mobile advertising company Quattro Wireless - are being asked to comment on the deal by the FTC as part of a private probe into AdMob's acquisition.

While the request for declarations from Google's competitors doesn't guarantee that the Federal Trade Commission will take exception to the deal, it certainly indicates that it is taking an interest - and should the FTC find Google is running foul of anti-monopoly rules could seek to force the company to modify its agreement or discard the deal altogether.

As the details of the investigation are being kept private, it's difficult to comment on the FTC's likely decision, but the investigation is likely to be enough to keep some Google executives awake at night for the next few months.

The deal between the two companies, which was agreed back in November, was inked in order to give Google a powerful advertising platform to implement on its Android operating system - while also giving it income from devices on other platforms, including Apple's iPhone. With the purchase making AdMob the largest single mobile advertising company in the market - with an estimated 21 percent share when combined with Google's presence - it would be a blow for both companies if the FTC blocked the acquisition.

Neither the FTC nor Google have thus far commented on the investigation, other than for Google's Adman Kovacevich to confirm that his company is "confident that [the FTC will] conclude that the rapidly growing mobile advertising space will remain highly competitive after this deal closes."

Do you believe that Google should be left alone, or are its competitors right to ask the FTC to intervene in the acquisition? Share your thoughts over in the forums.