Officials are investigating how many times a suspected al-Qaeda operative - accused of plotting to attack the US with a radioactive bomb - entered Switzerland.
Hansjürg Mark Wiedmer, a spokesman for the Federal Prosecutor's Office, said Swiss federal police began their investigation in May alongside United States officials.
Jose Padilla, a 31-year-old US citizen, was arrested in Chicago on May 8 after travelling from Pakistan via Zurich.
US officials are holding Padilla under new US terrorism laws on suspicion of having been involved in the scouting of locations to set off so-called "dirty bombs".
The FBI describes the bombs as combining conventional explosives with radioactive material. While they do not produce a nuclear explosion when detonated, the weapons are designed to spread small amounts of radioactive or biological material over dozens of city blocks.
Always in sight
Wiedmer said Switzerland's investigation includes "determining the circumstances of one of several possible stays of the individual in Switzerland in connection with his transit via the Zurich airport".
He also said Swiss officials had been watching Padilla, 31, before he was arrested.
US attorney general, John Ashcroft, said Padilla met senior al-Qaeda officials several times in 2001.
Padilla has been described as a Catholic-raised former gang member who converted to Islam.
The FBI has said the man was under surveillance throughout his trip from Pakistan.
According to the New York Times, Padilla travelled to Switzerland more than once. "Padilla left Pakistan in early April and travelled from Switzerland to Egypt and then back to Switzerland," the paper said.
It also said FBI agents secretly boarded his flight from Zurich to the US to keep him under surveillance.
"Worried that Mr Padilla might disrupt the Chicago-bound flight, agents asked airline security personnel in Zurich to inspect his luggage carefully and his personal effects, including his shoes," the paper said.
swissinfo with agencies