Two Australians have been arrested for allegedly attempting to fly out of Portugal with six rhino horns valued at 400,000 euros ($A538,000) in their luggage.
The arrests last week came after a spate of rhino horn thefts from European museums this year.
Police inspector Rui Almeida said the men are suspected of belonging to an international ring involved in the illegal trade of rhino horns to China.
"We have indications they were not acting alone," Almeida told AP by phone.
He declined to elaborate because foreign police forces, as well as Europol and Interpol, are continuing the investigation.
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He said "dozens" of European museums have reported the theft of the horns in recent months, including the Museum of Natural History at the University of Coimbra, in central Portugal, in April.
Authorities say the horns, purported to have aphrodisiac and medicinal qualities, are more valuable than gold on the black market.
Rising demand, especially in Asia, and a crackdown on the illegal trade have made them extremely valuable.
Rhino horn is commonly used in powdered form by people who believe they can help cure serious diseases, or boost sexual prowess. Rhino conservation activists dispute those claims.
Almeida said the Australian men - a father and son, aged 63 and 31 - are not suspects in the University of Coimbra theft. He said those horns are still missing.
Europol and Interpol are helping Portuguese authorities investigate where the six seized horns came from.
Almeida said the men were intending to catch a plane to Ireland when they were detained and did not resist arrest. They cannot be named under Portuguese law.
They appeared before a judge last week but were released on bail because smuggling carries a maximum jail term of five years and suspects can be held in jail awaiting trial only if the maximum sentence is longer than that.
