The mother of an American detained in Iran pleaded in a Foxnews.com interview for the safe return of her son, saying she hasn't heard from him since he inadvertently crossed the border into Iran while on a hiking expedition in July.

Josh Fattal of Elkins Park, Pa., along with Americans Shane Bauer and Sarah Shourd, were apprehended by Iranian authorities on July 31 after they entered the Islamic Republic while on a one-week hike through Iraqi Kurdistan.

"We have not had a phone call home," Fattal's mother, Laura, told Foxnews.com. "They had no intention of going into Iran."

The Iranian government has twice acknowledged that the three Americans are being held for illegally entering the country. The Swiss Embassy in Tehran, which represents U.S. interests there, was granted consular access to the detainees on Sept. 29.

But no one has heard from the three Americans, despite assurances from the Swiss government that they are in good health and receiving proper care.

Relatives of the three hikers submitted a petition Thursday containing 2,500 names to the Iranian Mission to the United Nations, the Associated Press reports. The petition calls it an "unfortunate case" and asks Iran to release the hikers "as soon as possible."

Fattal said she last spoke with her son on July 5 and exchanged a series of e-mails with him on July 27.

"We haven’t heard from them in 75 days," she said. "This is a terribly long amount of time and we are concerned. We want them released as soon as possible."

She said she learned that her son had been detained in Iran when she got a phone call from the U.S. Embassy in Baghdad on July 31.

The U.S. State Department says it has been aggressively lobbying the issue with Iran, but it gave no word on whether Tehran had responded.

A U.S. official, speaking on condition of anonymity because of the diplomatic sensitivity of the matter, said the government is doggedly working behind the scenes with the Swiss government to secure the three hikers' release. The official said the issue was raised on Oct. 1, when the five permanent members of the U.N. Security Council and Germany met with Iranian officials in Geneva over nuclear proliferation negotiations.

The official said that U.S. Undersecretary for Political Affairs Bill Burns also requested information on the welfare and whereabouts of two dual nationals, American scholar Kian Tajbakhsh and Reza Taghavi, a 71-year-old American who has been imprisoned since May 2008 without a trial or formal charges. Burns also pressed Iranian diplomats on the whereabouts of Robert Levinson, a former FBI agent who disappeared while on a trip to Iran in March 2007.

U.S. officials, including Sen. Bill Nelson, D-Fla., said they have reason to believe Levinson, who was sent to Kish Island as a private investigator in connection with a cigarette smuggling case, may still be alive.

"I believe Bob Levinson is in Iran – and, now that Iran and the U.S. are resuming talks, I’m hopeful the Iranians will help find and release him,” Nelson told Foxnews.com in an e-mail statement Wednesday.

State Department spokesman Andy Laney indicated in an interview that there may be additional talks with the Iranians on the hikers' detainment by the end of the month.

"Certainly we are concerned about these three Americans," Laney said, adding that "there hasn't been any sort of mistreatment to my knowledge."

Iranian President Mahmoud Ahmadinejad, while visiting New York in September to speak to the United Nations General Assembly, said in an Associated Press interview that he would ask the country's judiciary to expedite the process and to "look at the case with maximum leniency."