Well that would make him a terrorist. There is a zero chance it would happen from routine police activity.
Fires gun at fleeing suspect, bullet hits _ -> deaths. Now, how does _ end up with 10 million deaths?
Well, the US Air Force for example has accidently dropped nuclear warheads on several occasions. They are designed to be really tough, but there is probably a non-zero chance a bullet could end up setting a defective one off.
Of course reality can be far stranger than fiction so I am unlikely to think of the most likely risk.
PS: The odds of terrorists being able to kill 10 million people tomorrow is so low it's really not worth considering.
First, getting a nuke while hard is not enough. Killing 10 million people takes either a really big nuke in just the right spot, or a large number of nukes.
Second, having a nuke is not enough you need to be able to correctly detonate on command which it which is harder than you might think.
Third, you need to transport it to a useful location as there not stored in the middle of city's.
Fourth, you need to decide using them is more useful than threating to use them.
Fifth, if your comparing them to US police then they need to target the US.
So, no it takes far more than picking one up to kill 10 million people.
PS: Simply killing lots of people is not that useful to a terrorist organization. Making nukes less useful than you might think.
China, US, India, Pakistan, Russia.....