CAIRO--In an unusual signal of willingness to hold security forces accountable for abuses, an Egyptian prosecutor ordered the arrests of four police officers in connection with the deaths of more than three dozen Muslim Brotherhood detainees more than two months ago, state media reported Tuesday. Meanwhile, suspected Islamist militants ambushed an Egyptian military convoy in the volatile Sinai peninsula with rocket-propelled grenades, security officials said. A soldier and a civilian were reported killed and seven soldiers hurt--the latest violent episode in the strategic peninsula, where the army has been battling what appears to be an increasingly sophisticated insurgency. The move to prosecute police officers in the deaths of 37 demonstrators who were arrested during Cairo protests in August was being closely watched as a potential precursor to greater scrutiny of the interim government's overall dealings with the Muslim Brotherhood. Human rights groups have cited the maltreatment of Brotherhood detainees as a key cause for concern that military-backed administration will not move ahead with democratic reforms. Some 2,000 members of the Brotherhood are behind bars, including deposed Islamist President Mohamed Morsi, who is to go on trial next month. Reports of beatings and torture of prisoners have filtered out, including detailed accounts by two Canadians who were caught up in the dragnet and spent seven weeks behind bars before being released earlier this month. The Interior Ministry had initially dismissed the outcry over the deaths of demonstrators as they were being transported to a Cairo prison, blaming the fatalities on fighting that broke out after the detainees tried to escape and seized a hostage. But a preliminary investigation found that they were asphyxiated by tear gas fired into the stifling-hot and tightly packed van where they were confined, according to security sources quoted by the state-owned Ahram website. The four officers were ordered jailed pending further investigation, it said. Authorities have shown little inclination, however, to revisit the much larger death toll -- nearly 1,000 -- that came in mid-August, when police and soldiers violently dispersed Morsi followers who had set up sprawling protest camps. Egyptian authorities have reacted angrily to international criticism of the harsh crackdown on the Brotherhood, whose members they describe as terrorists. The Brotherhood has been formally banned and its assets are being confiscated. Coinciding with the campaign against the Brotherhood has been a major military offensive in the Sinai, targeting armed Islamist factions, some of whom are said to be from outside Egypt. The army and the interim government say Morsi allowed Islamist militant groups free rein in the peninsula before he was toppled in July. Egyptians, and neighboring Israel, say the offensive is a much-needed step to restore security. But tribal leaders have reported indiscriminate targeting of civilians amid the fighting. Last month, a Sinai-based journalist was given a six-month suspended sentence by a military court after being accused of lying about military actions.