Police are investigating whether protesters are criminally liable for covering a police motorcycle with handbills and blowing a whistle by an officer's ear. The incidents happened at the July 1 march. Photos of the two incidents have been widely circulated on the internet. They have stirred debate about the behaviour of protesters. One picture shows a police motorbike covered with protest handbills. Another shows an officer shrinking back as a man blows a whistle close to his ear. The police said the cases were being investigated. No one has yet been arrested. A riot policemen's group said many officers were unhappy about the pictures. They felt insulted when protesters swore at them. 'It's basic manners to respect each other,' said Joe Chan Cho-kwong, chairman of the Junior Police Officers' Association. 'We are used to all kinds of shouting and offensive words when it comes to triad crimes, but when they come from an average person, we can hardly accept that.' He welcomed the investigation. But one observer said such a probe could stoke further tensions between officers and protesters. Professor Ma Ngok, a political scientist at Chinese University, said public anger at the police stemmed from the increasingly heavy-handed tactics they used in rallies. The situation deteriorated after Andy Tsang Wai-hung became police chief, Ma said. 'It can become a spiral,' he said. 'When police deploy more officers during a rally, protesters become more agitated, and then the police become even more heavy-handed.'