Well, it's supposed to be from the earliest formation of our solar system. Hydrogen is the most common element by far in the entire universe, followed by helium, lithium, beryllium...etc. Complex chemical combinations are extremely rare. It's not likely that cyanide will be there. No virus's or bacteria or anything like that, much less a virus or bacteria that could do anything to this planets biological life. Also the extreme vast number of any possible "creatures" brought to this planet from somewhere else would die immediately if not very soon after.
The original personnel that collected the probe had on protective suits just in case. But they have obviously seen no need to continue that safeguard.
I think that they know what they are doing. If they can shoot up a rocket, approach the comet, collect samples, and not only bring it back to Earth nor not even back to a land area on Earth, but back to land on our very country's soil? They know what the heck they are doing.