The ABC effect, an intriguing low-mass enhancement in the invariant mass spectrum, is known from inclusive measurements of two-pion production in nuclear fusion reactions to the few-body systems d, He and He. It was first observed 1960 by Abashian, Booth and Crowe in the inclusive pd He X reaction. Its explanation has been a puzzle since then. In an effort to solve this long-standing problem by exclusive and kinematically complete high-statistics experiments, we have measured the fusion reactions to d, He and He with WASA-at-COSY. These measurements cover the full energy region, where the ABC effect has been observed previously in inclusive reactions. In a recent kinematically complete measurement of the reaction we have shown that the ABC effect in this basic double-pionic fusion reaction is correlated with a narrow structure in the total cross section with quantum numbers I(J) = 0(3), a mass of 2.37 GeV and a width of about 70 MeV. The mass is about 90 MeV below 2 times the mass of , the mass of a system, and the width is three times narrower than expected from a conventional t-channel process. In the double-pionic fusion reaction to the helium isotope He again the ABC effect is observed to be correlated with the appearance of a resonance-like structure in the total cross section at the same excess energy. From a previous exclusive experiment at CELSIUS-WASA it is known that the double-pionic fusion to He also exhibits a pronunced ABC effect. New data from COSY on the He reaction scanning the full ABC region are presented as well as the status of measurements in other reaction channels, where the new resonance might contribute.