(CNN) -- Mexican authorities searching a man with a bulge under his shirt at the airport in the nation's capital found 18 monkeys hidden beneath his clothes, police said.

Investigators grew suspicious after Roberto Sol Cabrera Zavaleta, 38, became "markedly nervous" when asked what he was transporting, Mexico's Public Safety Department said.

Two of the tiny titi monkeys he was carrying in a belt were dead, the department said in a statement, and 16 of them survived the journey from Lima, Peru.

Cabrera has been detained as authorities continue their investigation, the statement said.

In an interview with authorities released by police, Cabrera said he first carried the monkeys in his suitcase, but then hid the animals in his clothes so they would not be harmed by X-ray machines at the airport.

Cabrera described the animals as "pets" and told authorities he had purchased them for $30.

Titi monkeys are protected endangered species requiring a permit for possession, police said.

Images released by police show the tiny creatures, many of which are tied up in pouches, squirming in a cardboard box.