Former New Mexico governor Bill Richardson is back in the United States after failing to secure the release of an American contractor serving a 15-year prison sentence in Cuba for crimes against the communist state.

Richardson returned Wednesday after spending a week on the island trying to get permission at least to see 62-year-old Alan Gross.

The former governor, who has often served as a diplomatic troubleshooter in Cuba and North Korea, has said he was disappointed at how the trip unfolded. He speculated to reporters Cuba's stonewalling indicated the island's government had decided it does not want to improve ties with the United States.

But a Cuban official responded Wednesday, telling the Associated Press Richardson made the diplomatic maneuver “impossible” when he described Gross in interviews as “a hostage of the Cuban government.”

In an exclusive interview with the AP, Cuba's head of North American Affairs Josefina Vidal called Richardson's comments “slanderous.” She said they made it impossible for the government to grant Richardson's request to see the prisoner.

Vidal also said, despite media speculation, Cuba had never planned to release the jailed American during Richardson's visit.

Gross was arrested in December 2009 for bringing communications equipment into Cuba while working for a private firm contracted with the U.S. Agency for International Development. He was accused of distributing Internet equipment and satellite phones to Cuban dissident groups, but he says he was simply trying to improve Internet access for the island's small Jewish community.

Gross's family has sought his release on humanitarian grounds, saying his health has suffered and that his mother and daughter both have cancer. The U.S. has also called for Gross's release, with the White House describing his sentencing as “another injustice.”

The United States and Cuba do not have formal diplomatic relations, only interests sections that are technically part of the Swiss embassies in each other's capitals.