A roadside bomb in southern China killed two children who found the explosive wrapped in a package and began playing with it, Hong Kong media reported Thursday.

Before the blast Wednesday afternoon, residents in Shenzhen said they had received threatening phone calls and police thought the attack might be related to an unspecified grievance in the neighborhood, the Ming Pao Daily newspaper reported, citing unidentified sources.

Police declined to comment over the phone to The Associated Press on Thursday. Officials asked that all questions be faxed to them, and they did not immediately reply after the queries were sent.

Ming Pao reported that three boys found the bomb in a roadside flower bed and that they were playing with it when it exploded in Shenzhen, across the border from Hong Kong. The two children who died were 5 and 8 years old, and a 9-year-old boy was hospitalized, the paper said.

Hong Kong Cable TV interviewed an unnamed woman — wearing a yellow top stained with blood — who said she was the victims' mother.

"The two of them were playing in the sand," the woman said. "We were sitting in front of them. Then we heard a loud noise and I ran over to take a look. They were both laying on the ground there. Their faces were black."

The South China Morning Post quoted an unidentified hospital medic as saying that what appeared to be bomb fragments were found in the bodies. The blast blew a large hole in the ground and shattered car windows in the area, the English-language paper said.

A Hong Kong mass-market paper, Apple Daily, quoted local residents as saying that the neighborhood is popular with gangsters who frequently wage turf wars.