  We report on a high-statistics measurement of the basic double pionic fusion
reaction $pn \to d\pi^0\pi^0$ over the energy region of the so-called ABC
effect, a pronounced low-mass enhancement in the $\pi\pi$-invariant mass
spectrum. The measurements were performed with the WASA detector setup at COSY.
The data reveal the ABC effect to be associated with a Lorentzian shaped energy
dependence in the integral cross section. The observables are consistent with a
resonance with $I(J^P) =0(3^+)$ in both $pn$ and $\Delta\Delta$ systems.
Necessary further tests of the resonance interpretation are discussed.
