While that's possible, CNN reported sources in US intelligence service that tracked the firing of the missiles and said they came from close to the Russian border. It's not likely a Ukrainian air defense unit would have been out there by themselves, not with a Buk anyway.
I would take any reports coming out of US intelligence services with a bucket load of salt. Similar to Russia, US is hardly a neutral and reliable party in this. There is simply too much at stake for US right now on Ukraine. Obama has already used this incident to pressure EU on more sanctions.