LOS ANGELES (CBSLA.com) — An investigative report in the LA Times Sunday revealed that a quarter of all GPS devices used to track convicted criminals in LA County do not work.

The Times obtained a county probation department report which blames lousy batteries and false alarms on the ankle bracelets for the reasons many do not operate properly,

The radio tracking devices were employed to relieve crowding in prisons and jails by allowing felons to serve their sentences at home.

Among those possibly going unmonitored, The Times reported, were high-risk sex offenders, domestic abusers and violent gang members.

Similar findings were reported last summer in Orange County, prompting the cancellation of a contract with the private firm providing monitoring services there. LA County undertook an audit after the results were reported in Orange County, and The Times used a public documents request to obtain the LA data.

The Times reported that the private firm handling probationers, Sentinel Offender Services, has supplied devices with batteries that die too soon, and defective electronics that have generated excessive false alarms.

Sentinel countered untrained probation officers have not followed directions, and have mistaken dead batteries for faulty radio transmitters. The company also said many people being tracked by GPS are transients with limited access to electrical outlets.