Abstract: An amplitude analysis of the D-s1(2460)(+) -> D-s(+)pi(+)pi(-) transition is performed simultaneously in B-0 -> D-Ds+pi(+)pi(-), B+ -> (D) over bar D-0(s)+pi(+)pi(-), and B-0 -> D*D--(s)+pi(+)pi(-) decays. The study is based on a data sample of proton-proton collisions recorded with the LHCb detector at centre-of-mass energies of root s = 7,8, and 13 TeV, corresponding to a total integrated luminosity of 9 fb(-1). A clear double-peak structure is observed in the m(pi(+)pi(-)) spectrum of the D-s1(2460)(+) -> D-s(+)pi(+)pi(-) decay. The data can be described either with a model including f(0)(500), f(0)(980), and f(2)(1270) resonances, in which the contributions of f(0)(980) and f(2)(1270) are unexpectedly large, or with a model including f(0)(500), a doubly charged open-charm tetraquark state T-c (s) over bar(++) and its isospin partner T-c (s) over bar(0). If the former is considered implausible, the T-c (s) over bar states are observed with high significance, and the data are consistent with isospin symmetry. When imposing isospin constraints between the two T-c (s) over bar states, their mass and width are determined to be (2327 +/- 13 +/- 13) MeV and (96 +/- 16(-23)(+170)) MeV, respectively, where the first uncertainty is statistical and the second is systematic. The mass is slightly below the DK threshold, and a spin-parity of 0(+) is favoured with high significance. (c) 2025 The Authors. Published by Elsevier B.V. and Science China Press.