The Justice Department said they are only allegations and the burden is on the government to prove it.
A federal court in Washington issued a warrant to seize the tanker, the oil it carries and almost $1 million.
Following the Grace 1's release, Britain renewed its demand that Iran release the British-flagged tanker it seized in the Strait of Hormuz on July 19.
Accusing the ship of violations of bank fraud and money laundering laws, the International Emergency Economic Powers Act, and terrorism statutes, the United States complaint alleges "multiple parties affiliated with the IRGC" used voyages like that of "Grace 1" to support illegal activities.
The U.S. alleges a scheme to unlawfully access the U.S. financial system to support shipments to Syria by Iran's Islamic Revolutionary Guard Corps.
"We operate on the basis of complying with our own legal obligations", he said.
Gibraltar will "respectfully" deal with any application coming from the U.S. to open a separate proceeding to seize the tanker, Picardo told AP, adding that the government was not aware of the next logistical steps of the Grace 1.
The Gibraltar Chronicle said the ship was waiting for six new crew members including a captain to arrive on Sunday before it sets off.
"We have deprived the Assad regime in Syria of more than $140 million worth of crude oil", Gibraltar chief minister Fabian Picardo said.
Despite all pressures from the United States which seeks to drive Iran's oil sales to zero, a court in Gibraltar announced this week it was releasing the supertanker.
But Iran denied it had provided any assurances to secure the ship's release, saying Gibraltar was only seeking to "save face".
The authorities in Gibraltar also released the captain of the Grace 1, who is Indian, and three officers who had been arrested and bailed without charge pending the outcome of the investigation.
United States attorney for the District of Columbia, Jessie Liu, said: "A network of front companies allegedly laundered millions of dollars in support of such shipments".
Washington's move had delayed the court decision on the vessel's fate but Judge Dudley said in his ruling that he had not received a written request from the US.
The capture of the tankers heightened tensions just as European nations scramble to try to save a landmark nuclear deal with the Islamic Republic after the USA pulled out of the accord in May past year and started imposing sanctions on Iran.
Tehran condemned Thursday's US bid to block the release of the ship. By withdrawing from it, he unilaterally reimposed USA sanctions against Iran, hobbling its economy in a so far unsuccessful effort to force Iran to negotiate a new deal.
While the cargo was originally bound for Syria, Iran has provided assurance that this is no longer the case, the Gibraltarian government said in a statement on Friday.