The detainees' lawyer said it was doubtful they would be allowed to leave with their mothers but he wouldn't rule anything out.
Masoud Shafii said that from a legal point of view, "this possibility is very unlikely because the file is still in the investigation stage. A court's verdict can allow them to go, but speaking out of a legal context, anything can happen".
He stressed that the three have not formally been chargedwith espionage and such allegations have come only from "non-judicial officials," such as the intelligence minister.
"Basically, remarks made by non-judicial persons are pre-judgmental," he said.
Their families have said the three were hiking in the Kurdistan region of northern Iraq when they were detained and that if they did cross the border with Iran they did so unintentionally.
Since their arrest, relatives have had little news on their statuses. Their mothers were eager to talk with them and gauge where their health stands after months of captivity in Iran's Evin prison.
The mothers have appealed to Iranian authorities to release the trio from detention
The mothers, who were granted visas in what Iranian officialscalled an "Islamic humanitarian gesture", arrived in Tehran on Wednesday to secure the release of their children.
The mothers are hoping to meet with officials involved in the case, as well as with Iranian leaders Mahmoud Ahmadinejad, the president of Iran, and Ayatollah Ali Khamenei, the supreme leader, who has the final say in all state matters.
"We hope we're going home soon, maybe with our mothers," Josh Fattal told reporters after greeting his mother.
Their lawyer said the six would stay together at least until evening, but it was not clear whether the three detainees would have to return to the prison for the night.
Sarah Shourd said it had been "terrible to be away from our families for this long. We've only received one phone call and it was five minutes long".
But Shourd said their treatment by the Iranian authorities had been "decent" and that loneliness was the hardest part of her detention.
"Shane and Josh are in a room together, but I'm alone and that's the most difficult thing for me," she said.
Nora, Shourd's mother, has said she is especially worried about the effect that near-solitary confinement may be having on her daughter.
Swiss diplomats, who represent US interests in Iran, have reported that Sarah was suffering a serious gynecological condition, while Bauer had a stomach ailment.
But Heydar Moslehi, Iran's intelligence minister, on Thursday defended the treatment of the Americans.
"We have treated the US nationals according to our religious principles and on humanitarian grounds, even though these individuals committed an act of espionage by illegally crossing the border into Iran," Moslehi told Press TV.
The case could face complications from Iran's diplomatic showdown with the US and its allies.
Just before the mothers' arrival in Tehran, the United States said it had won support from other major powers for a new set of sanctions against Iran over its disputed nuclear programme.
The US, which has not had formal diplomatic relations with Iran since the 1979 Islamic Revolution, and its allies accuse Tehran of seeking nuclear weapons. Iran insists its nuclear programme is for peaceful purposes.