Thousands of US sex offenders, prisoners on parole and other convicts were left unmonitored after an electronic tagging system shut down because of data overload.

BI Incorporated, which runs the system, reached its data threshold - more than two billion records - on Tuesday.

This left authorities across 49 states unaware of offenders' movement for about 12 hours.

BI increased its data storage capacity to avoid a repeat of the problem.

Prisons and other corrections agencies were blocked from getting notifications on about 16,000 people, BI Incorporated spokesman Jock Waldo said on Wednesday.

In retrospect, we should have been able to catch this Jock Waldo , BI Incorporated spokesman

"At 0729 Mountain Time [1429 GMT] on 5 October, BI Incorporated experienced a problem with one of its offender monitoring servers that caused this server's automatic notification system to be temporarily disabled, resulting in delayed notifications to customers. The issue was resolved approximately 12 hours later at 1925 [0229 GMT Wednesday]," BI said in a statement.

Tracking devices continued to record movement and gather information, but corrections agencies could not immediately view the data, BI said.

In Wisconsin, local police and probation agents held about 140 sex offenders at local jails until the GPS tracking system was restored.

The offenders - about 300 in the state, most of them sex offenders - were never aware they were not being tracked, state Department of Corrections spokeswoman Linda Eggert said.

"In retrospect, we should have been able to catch this," Mr Waldo is quoted as saying by the AP news agency.

Correction at 1800: An earlier version of this story incorrectly included a photograph of electronic tagging equipment which, it has been pointed out to us, was not involved in the system failure.