A new search for decays of heavy isosinglet neutrinos produced by neutral-current neutrino interactions in the CERN wide-band neutrino beam has been conducted by the CHARM-II Collaboration. We searched for heavy neutrinos created by scattering muon neutrinos on nucleons and decaying into mu(+) mu(-) v(mu). Production and decay of heavy neutrinos would appear as double events in the detector. Each double event candidate was tested wheather the measured quantities are compatible with the kinematics of this process. No event passed this test. The analysis is based on 2 x 10(7) neutral-current neutrino events collected from 1987-1991. The experiment is sensitive to heavy neutrinos in the mass range 0.3-2.4 GeV/c(2). The best limit for the mixing parameter is \U-mu i\(2) < 3 X 10(-5) for a mass around 2 GeV/c(2). The analysis improved by an order of magnitude previous results derived from wide-band neutrino beam exposures of the CHARM detector.