Watkins also sought care at a hospital for injuries to his upper torso sometime after the incident, and the story he gave to hospital staff "did not seem to mesh up with the injuries," Masterson said at a news conference Monday evening.
The patas monkey was found dead of blunt force trauma to the head and neck early Saturday morning. The death left zoo workers shocked and devastated, zoo director Steve Burns said, and the Crime Stoppers organization offered an award of up to $1,000 for information leading to the culprits' arrest.
Investigators had not had a chance to question Watkins extensively and have not revealed whether they think the zoo break-in was a prank that turned violent or something done with more sinister intent. But the police department and community are "angered and outraged over this senseless crime," Masterson said.
"The loss of this patas monkey has touched many lives, including our officers and investigators."
The zoo doesn't have surveillance video. Instead, security guards patrol the grounds whenever the zoo is closed.
It was a security guard who first ran across the crime, Burns said, coming across two men early Saturday morning one inside the zoo and one outside the perimeter fence near the primate exhibit. Both men fled, with one running into the interior of the zoo.
Investigators believe Watkins is the man who was seen inside the fence.
Burns and police were searching the grounds when Burns heard a groan and found the injured monkey outside of its exhibit, near the fence surrounding the zoo. They were able to get the animal into a crate and to the zoo's animal hospital, but the monkey died just a few minutes later of blunt force trauma to the head and neck.
An inventory showed none of the other animals was missing or harmed.
Police say Watkins was visiting Boise with friends over the weekend from his home in Weiser, an agricultural town about 60 miles away near the Oregon-Idaho border.
Court records show Watkins has been in trouble with the law before, including drug arrests. Police said they do not know whether Watkins may have been under the influence of alcohol or drugs at the time of the break-in.
Officers have spoken with the other man spotted outside the zoo but do not expect charges to be filed against him, Masterson said.
Crimes at the zoo are rare, Burns said.
"I've been here for 15 years, and I don't remember any cases where we've had a visitor intentionally or even accidentally injure an animal," Burns said. "People in Boise are usually pretty respectful. We were just saying the other day that we can't even remember the last time that someone was found inside the zoo after hours. The security guards do a really good job."
Burns said it will take a few weeks before he can decide if the remaining patas monkey will be sent to another zoo or if another patas monkey will be brought in as a companion. The animals are social and need to be around members of their own species.
The crime may have raised interest in the patas monkeys. A donation for the remaining patas monkey under the zoo's adopt-an-animal program came in over the weekend, Burns said.
The monkey exhibit remains open to the public, although zoo workers were keeping some of the larger garage-sized doors to the exhibit closed to keep down noise, and keepers were giving the remaining patas monkey a little more attention, Burns said. The zoo kicked off a fundraiser to build a new exhibit house for the primates in September.
"That primate house was built back in the 1960s and it's just time to update it and provide the animals with more space and things like that," he said.
For now, he said, zoo workers are just focusing on caring for the remaining 300 animals at the zoo.
"We're going to grieve for the animal and make sure the community's OK. But we're going to move on with the plans that we have and continue to take care of the animals. Boise's a really nice place to live, and usually this kind of stuff doesn't happen in Boise," he said.