China's antitrust authorities are prolonging a second investigation into Google's $12.5 billion merger with Motorola Mobility, the handset manufacturer said Monday in a SEC filing. 

The extended probe could mean a setback for the closing of the deal, which already has been approved by all other necessary regulators including the U.S. Department of Justice and the European Commission.   

Motorola warned that it "can provide no assurances as to whether or when the transaction will receive clearance."   

However, it said it continued to "work closely" with the Anti-Monopoly Bureau of the Ministry of Commerce People’s Republic of China (MOFCOM) and still expected the acquisition to close during the first half of this year.  

The Motorola buyout gives Google control over a top manufacturer of handsets using its Android operating system, and Motorola's massive 17,000-strong patent portfolio shores up the Internet heavyweight's legal defenses for Android, which has been beset by patent lawsuits. 

Google has said Motorola will still run independently once the deal closes, but that pledge was cast into doubt last month when reports surfaced that Google planned to oust longtime Motorola chief Sanjay Jha and install one of its own executives as president and CEO.  

The rumored executive shakeup has yet to come to fruition.