Electromagnetic E = mc2
One more time, facts are proving STEMPhysics' claim.
Although the electromagnetic origin of the famous formula has been obviously demonstrated by Poincaré, Hasenörl and Planck, an intricate fate attributed the formula to Einstein's relativity which rapidly blurred its connection with electromagnetism.
As though everything has been orchestrated to hide the obvious evidence to public
How mass and electromagnetism are tied
The relativistic formula E = m.c^{2 } has nothing to deal with relativity principles. It has been long established upon the only electromagnetic equations.
Poincaré's demonstration
At the early 20^{th }century, Poincaré tried to explain the unbalanced energy carried by the electromagnetic Lorentz's force. The latter apparently violates the principle of action and reaction. In order to agree this principle, needs Poincaré to associate an electromagnetic energy to mass such as any variation of mass corresponds to a variation of electromagnetic energy:

dm = dE/c^{2 } (1)
Hasenörl's demonstration
In 1904, Hasenörl demonstrates how a cavity containing electromagnetic waves has an equivalent mass bound to the total electromagnetic energy:

M = E/c^{2 } (2)
Planck's demonstration
Few months later, Planck considered an object that radiates electromagnetic waves. Each propelled wave has a wellknown momentum equals to E/c and an equivalent mass of E/c^{2 }. Because the radiating object doesn't gain energy from anywhere, its mass can be considered as if it were able to provide a total electromagnetic energy:

E = M.c^{2 } (3)
The Synergetics evidence
All of these relationships would have alarmed physicists about the real nature of space and matter. However, by a kind of firm resolution, no physicists would have attempted to associate mass and energy to something immaterial (Poincaré seemly thought of it but didn't dare to claim it).
Synergy: a new concept for Physics
Synergy: the whole and absolute energy quantity that fulfill a volume at rest in a local reference space medium.
Establishing the genuine electromagnetic nature of everything and considering the interaction capability between a material system and its surrounding space, one fundamental equation of STEMPhysics is:

S = M.c^{2 } = M/( ε.μ) (4)
Were:

S is the synergy of the material system equals to the total energy of the whole interactions with its surrounding space medium.

ε and μ are the electromagnetic properties of the surrounding space. Both of them have no reason to be told constant.
Furthermore Kinetic energy doesn't need Lorentz's transforms
Yet another time, the formula of kinetic energy has been claimed to be a proof of the equivalence principle of relativity. Moreover it's still really not worth to introduce weird and complicated spacetime geometry.
From the primary law of inertia
The below equation is explained in a
precedent article.

dp.v = dE (5)
According Synergetics, (5) is now more fairly expressed as:

dp.v = dS = d(m.c^{2 }) (5')
According the surrounding space is homogeneous, c can be regarded as a constant and then:

dp.v = c^{2 }.dm (6)

m.v.dp = c^{2 }.m.dm (7)

p.dp = c^{2 }.m.dm (8)
By integrating the primary law of inertia
Equation (8) becomes:

p^{2 } = c^{2 }.m^{2 } + Constant (9)
According p is null at rest, (9) yields the socalled relativistic equation of inertia:

m^{2 }.c^{2 }  p^{2 } = m_{0 }^{2 }.c^{2 } (10)
And then of course:

S^{2 }  p^{2 }.c^{2 } = S_{0 }^{2 } (11)
Reaching the speed of light
The surrounding space remaining homogeneous, it is then the mass of inertia which evolves with speed. The wellknown equation (11) actually allows retrieving the relationship:

m.(1  v^{2 }/c^{2 })^{1/2 } = m_{0 } (12)
Note: yet another time mathematics confused physicists, because in the Synergetics expression of the mass (12), v can reach the speed of light. Hence, a system that moves at this speed has consequently a null mass at rest. This is on purpose the plain case of photons.
Discover what can be inferred from the electromagnetic energy of space