Ingested (as opposed to aspirated) mercury is not very toxic. Mercury compounds are.
https://en.wikipedia.org/wiki/Mercury_poisoning
The FBI may have detected emails or social media chatter where the budding Copernicuses of Islamic extraction working at the lab discussed stealing a truck load of the stuff and dumping it into Lake Meade, or other nefarious schemes.
Where it would go right to the bottom. And it would be sucked up by a vacuum in about a week.
Good plan, right there. /ha ha.