Boiler explosion last April in St Louis, anyone know the exact cause?

from CSB report:
On the day of the incident, there was a steam explosion inside
the Loy Lange building. A steel vessel weighing about two
thousand pounds--referred to as a Semi Closed Receiver or
SCR–launched through the air. The damage caused by the force
of the steam explosion left one Loy Lange Box Company
worker fatally injured and another in critical condition.

The SCR pulled loose from all the piping and floor attachments,
and rocketed up through the Loy Lange building. It went
through the roof, traveling at almost 120 mph. It rose to about
425 feet above street level and traveled laterally across
approximately 520 feet, and it remained airborne for over 10
seconds.

The energy released was equivalent to about 350 pounds of TNT.

In 2012, the SCR was repaired when a leak was discovered in
the bottom of the tank. It is important to emphasize that at that
time the entire 30" diameter elliptical bottom portion of the tank
was not replaced. Instead, a 24" center section was removed and
replaced with a custom-formed 24" diameter piece, called a tank
circle.
The remaining 6 inch ring of the original part of the tank—left
behind during the repair on the SCR—connected the new tank
circle and the original SCR shell.

An area of continuing interest surrounds a proposal received by
Loy Lange twenty five days after the repair was completed from
the same contractor responsible for the leak repair work. The
proposal suggested a replacement of the entire bottom of the
SCR to provide additional thickness in order to protect against against corrosion. This replacement ultimately did not occur, but we plan to conduct a more thorough examination of the details of that proposal.

On Friday, March 31, 2017, three days prior to the incident, the
employees again noticed a leak from the bottom of the SCR.
A cell phone was used to photograph that leak. The photo
showed leaks coming from at least two distinct sections of the
remaining 6 inch ring. The steam generation system was shut
down after the photograph was taken, and Loy-Lange contacted
a repair service provider. Plans were made for the repair
technician to arrive in the afternoon on the next business day,
Monday, April 3.

The explosion occurred in the morning of April 3.

Good question, I found another CSB report, as far as I can tell the investigation is on going and not considered closed. This has a more detailed scenario and equipment layout.

http://www.dli.mn.gov/ccld/PDF/boiler_incident_pv.pdf

This is going to be a legal nightmare because the city of St Louis has an ordinance requiring them to inspect all SCR's and related equipment which they never did, the city also requires a permit pulled for the repairs of the SCR which was never done, I think in the end there may be too many players who screwed up for charges to be filed, it will just go to Civil court, like the CO poisoning in Vermont years ago at a college, everyone from the landlord to the HVAC co to the installer to the city screwed up.

Well the contractor that did the repair must have stated it was temp and 25 days later submitted a new proposal to do more thorough work adding 50% to the original thickness, it was never approved, they do have an issue with not pulling a permit for the repairs. It was an emergency repair and most places will give you 48hrs to pull after the fact....oops

It was almost the equivalent of a small car, CSB estimated the blast to have equaled @ 300lbs of dynamite, not sticks, pounds.