We study the evidence for spin liquid in the frustrated diamond lattice antiferromagnet CoAl2O4 by means of single crystal neutron scattering in zero and applied magnetic fields. The magnetically ordered phase appearing below TN 8 K remains nonconventional down to 1.5 K. The magnetic Bragg peaks at the q 0 positions are broad and their line shapes have strong Lorentzian contributions. Additionally, the peaks are connected by weak diffuse streaks oriented along the 111 directions. The observed short range magnetic correlations are explained within the spiral spin liquid model. The specific shape of the energy landscape of the system, with an extremely flat energy minimum around q 0 and many low lying excited spiral states with q 111 , results in thermal population of this manifold at finite temperatures. The agreement between the experimental results and the spiral spin liquid model is only qualitative, indicating that microstructure effects might be important to achieve quantitative agreement. Application of a magnetic field significantly perturbs the spiral spin liquid correlations. The magnetic peaks remain broad but acquire more Gaussian line shapes and increase in intensity. The 1.5 K static magnetic moment increases from 1.58 amp; 956;B Co at zero field to 2.08 amp; 956;B Co at 10 T. The magnetic excitations appear rather conventional at zero field. Analysis using classical spin wave theory yields values of the nearest and next nearest neighbor exchange parameters J1 0.92 1 meV and J2 0.101 2 meV and an additional anisotropy term D amp; 8722;0.0089 2 meV for CoAl2O4. In the presence of a magnetic field, the spin excitations broaden considerably and become nearly featureless at the zone center