Reuters reports that on Thursday morning, six Russians died and 39 foreign passengers were rescued when a tourist submarine sank off the Egyptian Red Sea resort of Hurghada.

The Red Sea Governorate said the submarine, named Sindbad, had 50 people onboard: 45 tourists of different nationalities from Russia, India, Norway, and Sweden, and five Egyptian crew members. 

Four of the rescued tourists were reported to be in critical condition, receiving treatment in the intensive care units (ICU) of local hospitals.

The submarine, operated by Sindbad Submarines, “crashed at a distance of 1 km from the shore” at around 10 a.m., according to the Russian consulate in Hurghada. The vessel was on a regular underwater excursion to visit a local coral reef.

Sindbad Submarines operates two “recreational submarines” in its fleet, capable of diving to a depth of 25 meters (82 ft) for 40 minutes, allowing passengers to explore “500 meters of coral reef and its marine inhabitant.”

The governor, Maj. Gen. Amr Hanafy, said “investigations by the authorities are underway with the vessel’s crew to determine the cause of the incident,” but he noted that the submarine held a valid operating license and the crew leader held the required “scientific certificates.”

Hurghada is a popular resort on Egypt’s Red Sea coast, roughly 480 kilometers (300 miles) southeast of Cairo. The company that runs the submarine tours claims to be well-established in exploring the region’s dramatic seabed.

Thursday’s forecast showed clear skies, light winds, and warm conditions. The temperature was around 30 degrees Celsius (86 degrees Fahrenheit) with winds under 10 kph, according to CNN meteorologists.

In November, a live-aboard dive boat, Sea Story sank, leaving 11 dead or missing and 35 survivors near Marsa Alam on Egypt’s Red Sea coast.

Thanks to Alaric Bond for contributing to this post.