Iran's investigation into the case of three detained Americans will take time, a senior official was quoted as saying by Fars News Agency on Monday.
Iran detained the trio after they strayed across the border from Iraq more than a month ago, according to Iraqi officials.
Deputy Foreign Minister Alireza Salari was quoted as telling the Swiss ambassador in Tehran, "Due to the sensitivity of the time and location of the detention of these individuals, security bodies are investigating the cause ... of their entry into Iran's territory, and this is a matter that will take time."
The United States, which has called for their release, does not have diplomatic ties with Iran and Switzerland represents its interests in the Islamic Republic.
A leading Iranian lawmaker has suggested the illegal entry of the Americans may have been related to unrest that erupted after Iran's disputed election in June.
The vote and its turbulent aftermath have added to tension between Iran and the West, already at odds over Tehran's disputed nuclear program, after Iranian officials accused the United States and Britain of inciting opposition protests.
Last month, the families of the detained U.S. citizens expressed hope they would be released soon and said Shane Bauer, 27, Sarah Shourd, 31, and Josh Fattal, 27, inadvertently strayed across the border on a hiking excursion in Iraq.
A security official in Iraq's autonomous Kurdish region has said they wandered across northern Iraq's border with Iran near Ahmed Awa and were detained.
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