Responsibility for the killing lies with the Syrian regime, whether their forces killed him, or he was killed by others – the Government has a duty of care towards its citizens.
Nadim Houry from HRW confirms that he was killed today while at a private residence. His son Marcel and a female activist wounded.

This is distressing evidence of his death:!

Previous news/reports: [click on the links below]
June 2011:
Interview with Syrian Kurdish politician Mish’al at-Tammo  21.7.2011
Damascus: Mish’al at-Tammu is free: KURDWATCH, June 3, 2011—On June 2, 2011, Mish’al at-Tammu, speaker of the Kurdish Future Movement in Syria, was released from custody in Damascus. At-Tammu was able to be released after the Decree No. 61 was issued on May 31, 2011 (the amnesty decree). He had been sentenced to three years in prison on May 11, 2009.

News and reports from Syria and Western Kurdistan (7/10.2011)
Editor: Khalaf Dahowd & Gabar Çiyan
Kurdocide Watch CHAK Documentation Center in cooperation with International Support Kurds in Syria Association [SKS]-UK