Motorists ride through haze from fires in Pekanbaru, Riau province, in Indonesia on Aug. 27.
JAKARTA, Indonesia–Two Malaysian plantation managers in Indonesia have been barred from leaving the country while police investigate their possible role in agricultural fires in Sumatra this summer.

The men are managers of PT Adei Plantation and Industry, a subsidiary of Malaysia-listed Kuala Lumpur Kepong based in Indonesia’s Riau province.

Kuala Lumpur Kepong is one of Malaysia’s largest palm oil planters. The company has denied wrongdoing in the summer fires, which peaked in June.

Amran Aris, head of immigration in Pekanbaru, the capital of Sumatra’s Riau province, on Friday named the men as Tan Kei Yoong and Danesuvaran KR Singam. He told reporters that his office placed a six-month travel ban on the men earlier this month following a request from provincial police.

Attempts to reach the men for comment at their company were unsuccessful.

Adei was one of eight companies named as suspects following an outbreak of hundreds of illegal agricultural fires in Sumatra in June that sent pollution to record hazardous levels in neighboring Singapore and Malaysia. Clearing land by fire is illegal for all but the smallest landholders in Indonesia, but major fires are nonetheless an annual event in the dry season.

Environmental experts say many are caused by medium-sized companies clearing land to make way for new plantations.

In September, provincial police said they had found evidence that Adei burned peatland areas to expand its oil palm crops — a charge denied by the company –and that they were investigating precisely who at the company allegedly was responsible.

After the peak of the blazes, Indonesia’s environment ministry named companies that were under investigation—a rare move given a lack of major prosecution ever. But since then, officials across the government have been tight-lipped about investigations.

A senior government official told The Wall Street Journal this week that officials outside of the police have been under instruction not to name companies that might be implicated in the fires. The official said a number of mid-size companies would likely be prosecuted, and that many are part of complicated supply chains that feed some of the world’s largest palm oil companies.

Indonesia’s $20 billion palm oil industry is the world’s largest. Riau province is the heart of the industry and base of operations for major companies from Indonesia, Singapore and Malaysia. It’s also home to Indonesia’s largest pulpwood companies.

–Joko Hariyanto in Jakarta contributed to this article