Neuralink’s disclosure last week that tiny wires inside the brain of its first patient had pulled out of position is an issue the Elon Musk company has known about for years, according to five people familiar with the matter.

The company knew from animal testing it had conducted ahead of its U.S. approval last year that the wires might retract, removing with them the sensitive electrodes that decode brain signals, three of the sources said. Neuralink deemed the risk low enough for a redesign not to be merited, the sources added.

The company said last week that the implant’s tiny wires, which are thinner than a human hair, retracted from a patient’s brain in its first human trial, resulting in fewer electrodes that could measure brain signals.