But now another group of scientists conducting a separate study on the same beam of neutrinos at Gran Sasso claims their findings "refute a superluminal (faster than light) interpretation of the OPERA result."
Rather than measuring the time it took the neutrinos to travel from CERN to Gran Sasso the second experiment, known as ICARUS, monitored how much energy they had when they arrived.
Physicists believe that travelling even slightly faster than light would cause the particles to lose most of their energy in the process.
But the ICARUS team's calculations, published online last weekend, seemed to show they arrived with exactly the amount of energy particles moving at light speed should have had – and no more.
Tomasso Dorigo, a CERN physicist, wrote on the Scientific Blogging website that the ICARUS paper was "very simple and definitive."
He said it showed "that the difference between the speed of neutrinos and the speed of light cannot be as large as that seen by OPERA, and is certainly smaller than that by three orders of magnitude, and compatible with zero."
Prof Jim Al-Khalili, the University of Surrey, who threatened to eat his boxer shorts if the original OPERA result was proved right, said: "Usually we see this effect when particles go faster than light through transparent media like water, when light is considerably slowed down.
"So these neutrinos should have been spraying out particles like electrons and photons in a similar way if they were going superluminal – and in the process would be losing energy.
"But they seemed to have kept the energy they started from, which rules out faster-than-light travel."