Bixente Lizarazu may miss Bayern Munich's training trip to Spain next week because of alleged threats from the Basque separatist group ETA.
The Bundesliga champions' trip to Marbella has raised new fears for the security of the French World Cup star.
Bayern's coach Ottmar Hitzfeld revealed that Lizarazu might not travel with the team.
"If our training camp were in France, I'd see no problems," he said.
"Spain is a different security situation. We have to talk to the police."
The special anti-terrorist section of the Paris prosecutor's office last week opened an investigation into a threatening letter sent to Lizarazu, allegedly from ETA.
Security around the talented wing-back was tightened in December when the French police's special anti-terrorism division opened its own inquiry about the letter.
French newspapers last month printed large sections of the letter, which was received by Lizarazu's parents, who live in the French town of Hendaye on the border with Spain in France's Basque country.
The letter, written in Basque, demands "financial aid for the Basque country," but apparently does not specify a sum.
"You have been well-paid to wear the shirt of an oppressor state with money stolen from the Basques and the Basque regions," said the letter.
"Given the payments you have received from the enemy, ETA is writing to you," it added. "A lack of response to our request would lead to a response against you, or your property."