Police have put together a profile of the culprit in last week's explosions at two Beijing universities, and established how he built the bombs that injured nine people. A reward of up to 300,000 yuan (HK$282,600) is being offered for information on the blasts at Tsinghua and Peking universities, according to an internal document distributed to Tsinghua officials yesterday. The male suspect is aged around 20. He is 170cm, and looks like a student or worker. The document says he went to a car-parts shop near one of the universities on February 23 to buy spark plugs. He also used Tianjiao-brand crystal watches to make the bombs, the document says. It estimates that the bomber used a 300-gram powder charge at Tsinghua and a 500-gram charge at Peking University. Triggered by timing devices, the bombs went off behind radiators at about 11.40am on February 25 in Tsinghua's Heyuan dining hall and an hour later in the Nongyuan dining hall at Peking. A Tsinghua staff member said police had made the case a priority but there was no way to say whether officers were close to an arrest. A source at Tsinghua said the police found the suspect's detonator on site and checked all nearby stores selling car parts until someone reported a customer who did not look like a mechanic. 'They did their homework, got all the pieces of the case,' the source said. The Communist Party office at Tsinghua distributed the document to all campus departments. Students said yesterday that they had not seen the suspect's description but were happy to hear of the police's progress. 'I wouldn't be surprised if it's a student. Lots of things make people dissatisfied here,' said Tsinghua student Lu Jie, noting last year's case of a student from his school who threw acid on bears in Beijing Zoo. He said the bomber might have chosen Tsinghua and Peking universities because the campuses let anyone walk in. Except for random identification checks at the gates and more guards on campus, both have returned to normal, students and staff members say.