Investigators in Colorado found no evidence that a Taco Bell employee placed rat poison in a man’s food, police said Friday. Police made the announcement after reviewing video footage from the Denver-area location where the man bought the food Sunday and later fell ill, the Arapahoe County Sheriff’s Office said in a statement.

“Although there is no evidence that the employees at Taco Bell put the rat poison in the food, investigators can not account for how it got in the food,” the sheriff’s office said.

The unidentified man was taken to a hospital Sunday after he took a bite of a soft taco he brought home from the eatery and “felt a burning in his mouth and began to vomit,” according to the statement. Sheriff’s deputies at the hospital saw a “greenish-gray substance” in the taco, and lab tests confirmed the presence of rat poison in the food, the sheriff’s office said.

Earlier Sunday, the man argued with employees at the Taco Bell after he ordered a soda with his soft tacos and he was told the soda dispenser wasn’t working, the sheriff’s office said. The man argued with workers until he was given a burrito to replace the drink.

The sheriff’s office said investigators have been trying to reach the man on the phone for additional information without any success. On Thursday, no one answered the door at his home, the sheriff’s office said.

“If the customer has any information that can help with the investigation into how the poison could have gotten into the food, we request he contacts the Sheriff’s Office,” the sheriff’s office said.

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Restaurant manager Lary Swift told CBS Colorado earlier this week the man is a regular patron who’s caused problems before. On one occasion, she said, he threw a taco at an employee.

Sheriff’s Deputy John Bartmann declined to release the man’s identity to CBS Colorado, but he said enough rat poison was found in the food that “it would have been very serious to our victim’s health.”

Deputies closed the restaurant on Sunday evening.

Swift said police came back asserting that the eatery had “poisoned somebody.”

“We don’t carry poison in the restaurant,” she said. “We didn’t do anything like that. It didn’t even add up. It’s ridiculous.”

Swift said the restaurant turned over surveillance video to sheriff’s office investigators.

CBS News Colorado contacted Taco Bell’s corporate communications via email and received the following response: “The safety of customers and team members is a priority. The franchisee who owns and operates this location has informed us that they are working with local authorities in their investigation.”