  Observations of the core of the massive cluster Cl 0024+1654, at a redshift
z=0.39, were obtained with the Infrared Space Observatory using ISOCAM at 6.7
mum and 14.3 mum (hereafter 15 mum). Thirty five sources were detected at 15
mum and thirteen of them are spectroscopically identified with cluster
galaxies. The remaining sources consist of four stars, one quasar, one
foreground galaxy, three background galaxies and thirteen sources with unknown
redshift. The ISOCAM sources have best-fit SEDs typical of spiral or starburst
models observed 1 Gyr after the main starburst event. The median infrared
luminosity of the twelve cluster galaxies is 1.0x10^11 Lsun, with 10 having
infrared luminosity above 9x10^10 Lsun, and so lying near or above the 1x10^11
Lsun threshold for identification as a luminous infrared galaxy (LIRG). The
[OII] star formation rates obtained for 3 cluster galaxies are one to two
orders of magnitude lower than the infrared values, implying that most of the
star formation is missed in the optical because it is enshrouded by dust in the
starburst galaxy. The counterparts of about half of the 15 mum cluster sources
are blue, luminous, star-forming systems and the type of galaxy that is usually
associated with the Butcher-Oemler effect. Dust obscuration may be a major
cause of the 15 mum sources appearing on the cluster main sequence. The
majority of the ISOCAM sources in the Butcher-Oemler region of the
colour-magnitude diagram are best fit by spiral-type SEDs whereas
post-starburst models are preferred on the main sequence, with the starburst
event probably triggered by interaction with one or more galaxies. Finally, the
mid-infrared results on Cl 0024+1654 are compared with four other clusters
observed with ISOCAM.

Astrophysics