The men have allegedly told investigators they tried to connect with al-Qaida-linked militant groups in Pakistan and were intending to cross the border into Afghanistan and fight U.S. troops there.
The men were said to have used the social networking site Facebook and Internet video site YouTube to try to connect with extremist groups in Pakistan. When they arrived in Pakistan, they took that effort to the street.
They were reported missing by their families in the Washington, D.C., area a week ago after one of them left behind a farewell video showing scenes of war and casualties and saying Muslims must be defended.
Pakistani police detained them this week - along with one of their fathers - in Sargodha, a town in the eastern province of Punjab.
Regional police chief Javed Islam said the men had yet to be charged with any crime but they would "most probably" be deported. He declined to say how long police could hold them before they were charged.
A senior government official in Punjab said the five were being questioned first, and the overall legal process could take weeks.
"They are under investigation. We need to establish their links," Punjab Law Minister Rana Sanaullah told The Associated Press. "We are getting information that they had plans to travel to the tribal areas. We need to know which people they wanted to see and what their contacts were."
U.S. officials, including some from the FBI, have visited the men in custody.
The case has fanned fears that Americans and other Westerners - especially those of Pakistani descent - are traveling to Pakistan to join up with al-Qaida and other militant groups. It comes on the heels of charges against a Chicago man of Pakistani origin who is accused of surveying targets for the 2008 terrorist attacks in Mumbai, India.
Police have said those detained included three Pakistani Americans, two Ethiopian Americans and an Egyptian American named Ramy Zamzam who is a dental student at Howard University. The others were identified as Waqar Hussain Khan, Umer Farooq, Ahmad Abdul Minni and Aman Hassan Yamer. Pakistani officials have given various versions of their names and the spellings could not be confirmed.
Farooq's father, Khalid Farooq, also was detained. Pakistan police officials say the elder Farooq had a computer business in Virginia and shuttled between the U.S. and Pakistan. Investigators are still trying to establish what role - if any - he played in the men's alleged activities, officials said.
According to the Council on American-Islamic Relations, a Muslim advocacy group that helped bring the case of the missing men to the FBI's attention, the five left the country at the end of November without telling their families.
After the young men left, at least one phoned his family still claiming to be in the United States, but the caller ID information suggested he was overseas.
Islam, the police official, said Thursday the five men wanted to join militants in Pakistan's tribal areas before crossing into Afghanistan. He said they met representatives from the al-Qaida-linked Jaish-e-Mohammed militant group in the southeastern city of Hyderabad and from a related group, Jamaat-ud-Dawa, in Lahore, but were turned away because they were not trusted.
Pakistan has many militant groups based on its territory and the U.S. has been pressing the government to crack down on extremism. Al-Qaida and Taliban militants are believed to be hiding in the lawless tribal belt near the Afghan border.
In August, police arrested a group of foreigners, including a Swede who had spent time in Guantanamo Bay prison camp, close to the Afghan border region and publicly accused them of al-Qaida links. They were held for over a month before being released and put on a plane out of the country.
Associated Press writers Asif Shahzad in Islamabad and Babar Dogar in Lahore contributed to this story.