Antitrust regulators in Europe and the United States gave clearance last month to Google’s proposed $12.5 billion acquisition of Motorola Mobility, but the deal still hasn’t closed.

So what’s the hangup? China, it turns out.

Here’s the text of a regulatory filing from Motorola, made public this morning.

Upon agreement with Motorola Mobility Holdings, Inc. (“Motorola Mobility”) and Google Inc. (“Google”), China has extended the second phase of the investigation for the Anti-Monopoly Bureau of the Ministry of Commerce People’s Republic of China (“MOFCOM”) to review the pending transaction between Motorola Mobility and Google. Motorola Mobility and Google continue to work closely with MOFCOM to conclude its investigation and expect the transaction to close during the first half of 2012. The transaction has been investigated and cleared without conditions in all other jurisdictions with pre-closing clearance requirements. Motorola Mobility and Google can provide no assurances as to whether or when the transaction will receive clearance by MOFCOM.

The filing doesn’t explain the reason for the extension.

Despite clearing the deal, regulators in the U.S. and Europe both expressed concern about the potential impact on licensing of “standard essential” patents, amid questions about whether Google will do enough to make Motorola’s standards-related patents available to competitors on reasonable terms.

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