Police arrested the duo, aged 21 and 23, while they were on duty at the school about 11pm.
Sentul police chief ACP R. Munusamy said a police team picked up the suspects, following the discovery of a package containing a .38 revolver and a Rambo dagger on the ceiling of a guard post.
He said another security guard made the discovery at 10am Sunday and reported it to his employer and the police.
"Following this, the police picked up a 25-year-old man at his house in Selayang about 2am, and seized five cameras, machete, dagger, handcuff, gloves and a pistol box," he told a press conference here.
The third suspect is a telephone salesman. Based on the goods recovered, the three suspects were believed to be involved in burglary.
ACP Munusamy said the siblings were employed four months ago as security guards at the school but were often absent from duty, adding that one of them had a criminal record.
In another development, police arrested two men last Friday and recovered various items, including 16 computer laptops, several mobile phones and cameras worth a total of RM30,000.
The suspects were nabbed following a burglary at a house in Sentul Utama on May 5, after the crime was recorded by closed-circuit television cameras.
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