The families, based in the northern Virginia and Washington, D.C., area, were particularly concerned after watching what is described as a disturbing farewell video from the young men, showing scenes of war and casualties and saying Muslims must be defended.
"One person appeared in that video and they made references to the ongoing conflict in the world and that young Muslims have to do something," said Nihad Awad, of the Council on American-Islamic Relations, or CAIR. The video has not been made public.
On Thursday, police in Pakistan said the Americans had told investigators they came to the country to take part in "jihad," or holy war. Usman Anwar, the police chief of the eastern city of Sargodha, said they are "directly connected to al-Qaida," although he did not say what evidence he had to support the claim.
After the disappearance of the five men in late November, their families, members of the local Muslim community, sought help from CAIR, which put them in touch with the FBI and got them a lawyer.
The missing men range in age from 19 to 25. One, Ramy Zamzam, is a dental student at Howard University.
They were arrested Wednesday at a house in Sargodha linked to the banned militant organization Jaish-e-Mohammed, Pakistani officers said.
Regional police chief Javed Islam said the men wanted to join militants in Pakistan's tribal area before crossing into Afghanistan. He said they met representatives from the al-Qaida-linked Jaish-e-Mohammed in Hyderabad and from a related group, Jamat-ud-Dawa, in Lahore.
"They were asking to be recruited, trained and sent on jihad," Islam said, but were turned down because they did not have any "references" from trusted militants.
Islam added that investigators were sharing their findings with FBI officials who had arrived in Sargodha.
On the heels of charges against a Chicago man accused of plotting international terrorism, the case is another worrisome sign that Americans can be recruited within the United States to enlist in terrorist networks.
President Barack Obama declined to talk specifically about the case Thursday, but said, "We have to constantly be mindful that some of these twisted ideologies are available over the Internet."
Obama, in Oslo, Norway, to accept the Nobel Peace Prize, also praised "the extraordinary contributions of the Muslim-American community, and how they have been woven into the fabric of our nation in a seamless fashion."
A Virginia Muslim leader said the five men did not seem to have become militant before they left the U.S.
"From all of our interviews, there was no sign they were outwardly radicalized," said Imam Johari Abdul-Malik.
In Pakistan, police officer Tahir Gujjar did not identify them but said three are of Pakistani descent, one is of Egyptian descent and the other has Yemeni heritage.
Pakistan has many militant groups based in 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 lawless tribal areas near the Afghan border.
In Washington, a spokeswoman for the FBI's local office said agents have been trying to help find the men.
"We are working with Pakistan authorities to determine their identities and the nature of their business there if indeed these are the students who had gone missing," said the spokeswoman, Katherine Schweit.
According to officials at CAIR, 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 they were overseas.
A Howard University spokesman confirmed Zamzam was a student there but declined further comment.
Samirah Ali, president of Howard University's Muslim Student Association, said the FBI contacted her last week about Zamzam, and told her he had been missing for a week. Ali said she's known Zamzam for three years and never suspected he would be involved in radical activities.
"He's a very nice guy, very cordial, very friendly," Ali said.
One of Zamzam's younger brothers, interviewed at the family's Alexandria, Va., apartment, said Zamzam has a 4.0 grade-point average.
"He's a good guy," the brother said, identifying himself only by a nickname, "Zam." "He's a normal Joe."
Associated Press writers Eileen Sullivan and Matt Apuzzo in Washington; Asif Shahzad, Zaran Khan and Munir Ahmad in Islamabad; and Nafeesa Syeed in Alexandria, Va., contributed to this report.