Saudi Arabia on Monday urged its citizens to take extra precautions when travelling to Iran. The warning came after an insecticide poisoning at a hotel frequently used by Shiite pilgrims killed at least four Saudi nationals in Iran last week.

“The issue is still under investigation amid the lack of clarity and mysterious circumstances surrounding it, the Ministry of Foreign Affairs calls upon Saudi citizens traveling to Iran to take the utmost caution to preserve their safety and security as well as their families from any harm,” Saudi Arabia’s Foreign Affairs Ministry said Monday in a statement.

Thirty-three Saudi nationals reportedly came into contact with a poisonous gas at a hotel in the Iranian city of Mashhad last week. The poison claimed the lives of four of Saudi’s Shiite nationals, three children and one teenager, and injured 29 others, including women and children, according to Saudi Arabia’ Foreign Affairs Ministry.

Iranian officials told local news outlets Monday that 22 of the 23 remaining Saudis being treated for poisoning had been released from the hospital.

The Saudi Arabian government is seeking compensation for the victim’s families from the Iranian regime as well as full transparency on the investigation’s findings. So far, five people have been arrested in connection to the poisoning. The investigation was ongoing, but several Iranian officials have said the incident was an accident.

“It is a matter of time to definitely give verdict about the possible criminal nature of the poisoning; currently, five people have been arrested in relation to the incident; the hotel staff and other authorities have been interrogated, and the toxicological tests have been done on samples of the corpses,” Iran’s deputy of Chief Justice Gholamhossein Mohseni Ejei told reporters last week.

The incident occurred when the poison wafted into guestrooms in the hotel, after the hotel staff had used it as a pesticide, according to Agence France-Presse.

Saudi Arabia has also called on Tehran to protect its citizens who do travel the Shiite republic. Many of Saudi’s Shiite minority population travel to the religious city of Mashhad, which houses the shrine of the eighth Imam of Twelver Shiites, Imam Reza. It is one of the holiest sites in the country for Shiite Muslims. 

“The Ministry also urges the Iranian government to shoulder its responsibilities towards accelerating the results of the investigation with full transparency,” the statement added. “Along with the provision of the medical care for the injured and the provision of the necessary protection for Saudi nationals in Iran.”