A man arrested on suspicion of attempted murder after a three-year-old boy ended up in a crocodile enclosure has been released because he is not fit to be interviewed, police have said.

The 30-year-old man from Norfolk has been bailed while detectives from the major crimes unit conduct further inquiries, Cambridgeshire police said.

The force said officers were called to Johnsons of Old Hurst zoo in Huntingdonshire at 1.24pm on Thursday over “reports of an incident involving a three-year-old boy, during which he ended up in the crocodile enclosure”.

The boy was pulled out of the enclosure by staff from the zoo. He was taken to Addenbrooke’s hospital in Cambridge with serious injuries and is in a critical but stable condition, police said.

DI Verity McCann said: “Our inquiries are ongoing as we continue to understand the circumstances surrounding this distressing incident. Our thoughts remain with the boy, and his family and specialist officers continue to support them through this difficult time.”

The 30-year-old man, who is not known to the victim, has been bailed until 18 September.

A villager, who did not want to be named, told PA Media that the zoo owner’s wife, Tracey Johnson, jumped into the enclosure to try to save the child.

In a statement posted on social media, the Johnsons said: “Our thoughts and prayers are with the boy and his family following the incident that occurred today.

“Out of respect to the family, our tropical house will remain closed until further notice. If you have any questions about the incident, please direct inquiries to Cambridgeshire police constabulary.

“The rest of the site will remain open as normal.”

According to its website, the zoo is home to “over 100 fascinating animals” including African lions, Bengal tigers and crocodiles. The zoo’s two largest crocodiles are called Romeo and Cuddles, the zoo says on its Facebook page.

Charlotte Lowe, a Huntingdonshire district councillor, said she “can’t fathom” how the boy ended up in the crocodile enclosure.

Speaking outside the attraction on Thursday, Lowe, the councillor for Warboys ward, said: “I can’t fathom how it’s happened because they’ve got all the right protection and safety equipment, for want of a better word, in there.”

Ben Obese-Jecty, the MP for Huntingdon, said he had been communicating with senior officers, who were treating it as a critical incident.

“This is now a live criminal investigation and I would ask people to refrain from speculation online,” he said.

“My thoughts are with the young victim and his family during a hugely traumatic and difficult time.”