Just read a paper where a BSL-3 enhanced civilian laboratory created a strain of H5N1 with an estimated r-naught of 2-3 and a mortality rate of 60%. They essentially created a super lethal airborne version of avian influenza which demonstrated airborne transmission between ferrets. Ferrets are used in this type of research as they often contract the same respiratory viruses as humans and their lungs and airways bare a striking physiological similarity to human’s.
To put this in perspective COVID-19 has a r-naught between 2-3 and a mortality rate of 0.5-1%. So this is H5N1 these researchers created was estimated to be as transmissible as COVID-19 but two magnitudes higher in mortality. It would kill well over half the people who contracted it of any age. This lab was only BSL-3 not 4. So it was not even the highest grade of bio-safety and we have had multiple accidents from BSL-4 labs in recent years.
If it was released from this civilian lab via malice or negligence it would be hard to truly estimate the death toll, but it would be massive. Given the amount of this synthetic biology research going on now in relatively unregulated civilian labs across the country I would expect a man made outbreak to be highly probable in the near future.