An acid-filled beer bottle was dropped onto a street in the same area of densely populated Hong Kong for the third time in six months, police said Monday, fueling media speculation about a serial attacker.

Hong Kong has seen corrosive liquid hurled from buildings in a series of similar attacks the past year. Police arrested one in connection to a recent attack in December, but for the most part, the bizarre cases have mystified authorities. It's unclear what has motivated the attacks.

In the latest attack on Sunday, a beer bottle containing an acidic substance broke after being dropped onto an awning at street level in Hong Kong's Sham Shui Po district, with the liquid splashing onto the face of male passer-by, police said in a statement.

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"There was a bang, and people looked up and saw lots of smoke. And someone called the police. The bang was loud. The pedestrians ran away immediately," an unidentified female shopkeeper who works nearby was quoted as saying in the South China Morning Post Monday.

No one was immediately arrested, police said.

The injured man rinsed his face with water and left before officers arrived on the scene.

It was the third case of a plunging acid-filled beer bottle in Sham Shui Po in six months, according to the Ming Pao Daily News, sparking speculation about a serial offender. The same brand of beer bottle, Blue Girl, was used in all three cases, the newspaper said, writing in its story that "the Blue Girl corrosive bomb madman strikes again."

Police spokeswoman Candice Siu said officers are still investigating similarities between the three cases.

While none of acid attacks in Sham Shui Po have hit big crowds, similar incidents in other districts have, injuring dozens of pedestrians since December 2008.

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