The weekend heist in the northeastern city of Nelspruit was a blow to efforts to curb the clandestine trade in rhino horn, which has surged in recent years despite an increase in funding for anti-poaching efforts in South Africa, home to the majority of the world's rhinos.

Today, forensic investigators from the police analyzed the crime scene at the Mpumalanga Tourism and Parks Agency, and officials were exploring the theory that it was an inside job.

"It's too early to be ruling out that possibility," said Captain Paul Ramaloko of a South African investigative police unit called the Hawks. "We are approaching the investigation with an open mind."

The robbery of "nearly 40" rhino horns occurred on Saturday night, Ramaloko said.

The thieves targeted a safe where the parks agency of Mpumalanga province keeps some rhino horn stock temporarily before transporting it to "another undisclosed location" for longer-term storage, said Kholofelo Nkambule, an agency spokeswoman. The powerful machine tool used by the robbers broke part of the safe into pieces, she said.

Government agencies and some private game reserves keep stockpiles of rhino horns, including some confiscated from poachers and others shaved off rhinos in an attempt to deter attacks.

But the heist in Nelspruit raised concern about the security of those stashes in South Africa, which is considering whether to propose a regulated trade in rhino horn in order to curb poaching.

Legalisation would require the approval of CITES, the international body that monitors endangered species and will meet in South Africa in 2016.

"In terms of public institutions, we have never had such a case" of rhino horn theft, said Albi Modise, spokesman for South Africa's environment ministry.

Last year, 66 rhino horns worth several million dollars on the illegal market were stolen from a private wildlife reserve in South Africa's Limpopo province, according to local media. In that case thieves reportedly used a blowtorch to break into a safe where the horns were kept.