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The OPERA detector in Gran Sasso, Italy, is designed to detect neutrinos generated from protons 730 kilometres away at CERN, near the French-Swiss border. (OPERA)

Subatomic particles clocked at speeds exceeding the speed of light may have been going even faster than they appeared, physicists say.

A problem with some of the equipment used in the original experiment may have led to an overestimate of the time it took the particles, known as neutrinos, to make their 730-kilometre journey, reported CERN, the European Organization for Nuclear Research in a statement Thursday.

As a result, their speed may have been underestimated.

The problem was one of two equipment issues discovered by OPERA, the international physics collaboration that ran the experiment, CERN said.

A day earlier, CERN confirmed that a different equipment issue — a loose cable — may have led to the opposite problem. That is, it may have cause the neutrinos' travel time to be underestimated and their speed to be overestimated.

OPERA plans to run new measurements in May to test how the two problems may have affected the experiments, CERN said.

OPERA reported last September that they had clocked neutrinos travelling faster than light between the laboratories of CERN, the European Organization for Nuclear Research, near Geneva, and Italy's National Institute of Nuclear Physics, or INFN, in Gran Sasso. The results were contrary to expectations based on Albert Einstein's special theory of relativity

At the time of the announcement, the researchers said they were surprised by the finding but were unable to trace it to an error. They asked other scientists to scrutinize the results and independently verify the measurements at a different experimental site.

The new error reported on Thursday has to do with an oscillator, a device used to provide time stamps that are used to synchronize the GPS systems used in the experiment. The GPS and atomic clock measurements together are used to measure the neutrinos' travel time.