We systematically construct tetraquark currents of I G J P C = 1 − 1 ++ and classify them into types A (antisymmetric), S (symmetric) and M (mixed), based on flavor symmetries of diquarks and antidiquarks composing the tetra quark currents. We use tetraquark currents of type M to perform QCD sum rule analyses, and find a tetraquark current η M 5µ with quark contents qsqs (q = u or d) leading to a mass of 1.44 ± 0.08 GeV consistent with the a1(1420) state recently observed by the COMPASS collaboration. Our results support tetraquark explanations for both a1(1420) and f1(1420), assuming that they are isospin partners. We also study their possible decay patterns. As tetraquark candidates, the possible decay modes of a1(1420) are S-wave a1(1420) → K * (892)K and P -wave a1(1420) → f0(980)π while the possible decay patterns of f1(1420) are S-wave f1(1420) → K * (892)K and P -wave f1(1420) → a0(980)π. We speculate that a1(1420) is partly responsible for the large isospin violation in the η(1405) → f0(980)π0 decay mode which is reported by BESIII collaboration in the J/ψ → γ3π process.