The Special Task Force (STF) member used his officially issued automatic assault rifle to shoot himself at the gate of the Temple Trees residence while the premier was inside the sprawling complex.
Sri Lanka interviews 47 potential hangmen
It was not immediately clear why the commando took his life, but police officials said an investigation was underway.
An opposition legislator raised Thursday's suicide in parliament during a debate on the budgets of the military and the police, but there was no immediate comment from the government.
Sri Lanka's leaders and visiting dignitaries are given protection by the Special Task Force which is specially trained for security as well as urban guerrilla warfare.
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