CALGARY -- Police and Calgary Zoo officials are investigating after two men broke into the zoo early yesterday morning and were mauled by a 150-kg Siberian tiger.

The men, both 27-year-old Calgarians, are believed to have scaled a 2.5-metre fence topped with barbed wire north of the zoo's west gate about 1 a.m. and wandered to the tiger exhibit.

"Certainly I wondered what they were thinking," said zoo spokesman Grahame Newton.

"The tigers are behind those enclosures for a reason."

At 1:03 a.m., a security camera caught a two-second glimpse of one man, and at 1:07 a.m., the men were seen on camera receiving first aid at the zoo's security office.

What happened in those four minutes is still unknown, but investigators believe the men jumped over a one-metre fence and approached the tiger enclosure, potentially sticking their arms inside the holes in the wire toward Vitali, a two-year-old male.

Tiger curator Tim Sinclair-Smith, said Vitali's paws are too large to fit through the same holes, so at least one of the men likely reached inside the cage, where the tiger is presumed to have grabbed the suspect's arm, pulled it in, and mauled it.

"A tiger is a carnivore, they are going to behave naturally, this is his natural reaction," said Sinclair-Smith.

"Vitali has certainly done nothing wrong."

It is believed both men sustained injuries -- one by helping his friend pry away from the cage.

The men placed a call to the personal cellphone of a third friend, who happened to be an on-duty female security officer at the zoo, but whom police confirm did not provide the men access to the grounds.

One of the men remains in hospital with serious injuries -- believed to be bites and claw gouges -- in at least one arm. Police were still waiting to speak with the men yesterday afternoon, but said break and enter and trespassing charges may be laid.

They are still looking into whether or not alcohol was a factor in the case.

Sinclair-Smith said no zookeeper has ever been that close to Vitali, or the other two tigers, Vitali's mom, 14-year-old Kita, and sister, Katcha, nine, because they are so dangerous.

All three are fed through safety chutes so zookeepers remain at a good distance.

Dr. Sandie Black, head of veterinary services at the zoo, said in instances like these, the animals are often the ones who suffer, and following the investigation, Vitali could be stripped of certain privileges like being free to roam his enclosure at night.

Rosemarie Siever, of Edmonton, was visiting the zoo with her husband, Charles Macdonald, yesterday, and after hearing about the attack, said not breaching safety enclosures is common sense.

"He's a moron, that's all I have to say," she said.

Vitali was inspected for any injuries he may have sustained from the fence, but was given a clean bill of health and was back on display as of yesterday morning.