More than 10,000 broadband users were cut off by cable thieves who mistook an undersea fibre-optic cable for copper.
Services were restored just over a day later, and BT Openreach is offering a reward of up to £1,000 for information that leads to a conviction.
BT Openreach believes the damage was caused by metal thieves who thought the cable would contain valuable copper and left empty-handed.
A BT statement on Friday said: “We face a major engineering challenge to restore service to the affected communities and broadband users.
“The damage to the fibre cable was caused at low tide at Strathcarron, the tide then turned leaving the damaged area 20 metres out into the loch.
“The next low tide was 2pm today and we’re currently making a concerted effort to carry out a temporary repair with the help of local coastguards.”
The attackers struck on Thursday afternoon, damaging the cable on the banks of Loch Carron, which reaches communities in Skye, Lochalsh and the Western Isles.
The one-kilometre cable serves exchanges in Ardvaser, Balallan, Duntulm, Glendale, Glenshiel, Kilillan, Scalpay, Skeabost and Uig.
BT Openreach was able to restore services overnight to three exchanges via microwave radio link, but around 2,000 customers were left without phone or broadband, and broadband to almost 8,000 homes in the Northwest Highlands was also affected.
Engineers had to wait until Friday’s low tide at 2pm, and raced against time for six-and-a-half hours to restore services before the tide came back in.
BT uses a technology called RABIT to alert police and engineers to attacks on its network, and marks all copper cables with Smart Water, which can be detected by scrap dealers and police, and identifies where the cable was taken from.