Getting sentries to kill engies

Two engies were double humping one sentry.  They were perfectly sandwiched between the sentry and a dispenser, so I thought I would get them both killed by undisguising behind the dispenser.  One engy got blown up immediately.  The other engy shoved his wrench up my wazoo.

So the question is: Can a sentry only kill the original engineer who built it?  Is this true even if another engineer has upgraded it?  Or is it possible to kill both engineers with the same sentry?

A sentry can only kill it's owner.

I think only one get's hurt by his own bullets.

Scurve (I think) found out that friendly sentry rockets actually hurt everyone. But in that situation, do what you did and shoot the one you see not getting hurt.

Only your own sentry can hurt you, more importantly only your own buildings can stop your bullets.  Engineer teams don't do this, but the smart thing to do is put each others dispensers infront of the sentries, so now you have a dispenser shield that your sentry can shoot through.

Numenor speaks the thruth. There was a thread a while ago, and yes, a sentry can only kill The Engineer Who Built It.

Chro YOU ARE PRO

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Only if they're airblasted I think.

There is no friendly fire in TF2, including friendly sentry rockets (as long as they aren't from your own sentry).

Then Scurve has misinformed me

The sentry rocket FF happens sometimes, just not most of the time. I'd call it a bug.