At least 27 people died and six were wounded in the attack Friday.
RIU Hotels & Resorts said in a statement that it is collecting information about what happened on the beach in front of the Hotel Riu Imperial Marhaba in the Port El Kantaoui neighborhood of Sousse.
It said it is in "permanent contact with authorities."
RIU promised to report its findings to the public on its Website and through social media.
The company's statement made no mention of the nationalities of guests killed.
And, Rafik Chelli, one of Tunisia's top security officials, told Mosaique FM that the gunman shot by security forces after attacking a Tunisian beach resort was unknown to authorities.
He said the man, from the city of Kairouan, came from the beach hiding his Kalashnikov under an umbrella before opening fire on the tourists. From there he entered the Hotel Imperial through the pool, shooting people as he went.
Chelli said there were many foreigners among the victims but he did not have an exact count of the nationalities.
He said authorities had a plan to protect the hotels during the month of Ramadan but this was an isolated operation that is difficult to counter against and there is never zero risk.
A tourist from Ireland says she was on the beach in Tunisia with her two sons when she heard what she first thought were fireworks.
Elizabeth O'Brien tells Ireland's RTE radio she looked about 500 meters away and saw a hot air balloon collapse, then heard rapid firing and saw two people who were going to go up in the balloon running toward her.
O'Brien says she rushed to the sea to get her children and that waiters and security on the beach urged them to run as they raced toward the hotel. They ran to their room and holed up there.
She says her travel agent told her to go talk to someone at the reception desk but she's too afraid to leave the room.
British Prime Minister David Cameron says the government's crisis committee, COBRA, will meet later Friday in response to the attacks in France and Tunisia.
He offered condolences and "our solidarity in fighting this evil of terrorism."
"The people who do these things, they sometimes claim to do it in the name of Islam," Cameron said. "They don't. Islam is a religion of peace."
He said the attackers acted from "a twisted and perverted ideology we have to confront with everything we have."