Fletcher was indicted on murder and other charges in Tonga in the July 2016 death of his Canadian wife, Patricia Linne Kearney.
Earlier this year, a federal judge in Honolulu ruled that Fletcher could be extradited.
But a letter Tuesday from the State Department expressed concern that Fletcher would have faced a death sentence or life in prison and that he would not have received a defense attorney for free.
If he had been convicted and sentenced to death, Fletcher would have been hanged. He would have been the first person executed in Tonga since 1981.
Fletcher’s lawyer, Melinda Yamaga, declined comment on where Fletcher went following his release and said he did want to speak with journalists about his case.
Three diving operators saw Fletcher assaulting his wife on a dinghy after she picked him up at a Tonga port, according to provisional arrest documents filed in federal court in Honolulu.
Fletcher kept kicking and punching Kearney as the couple arrived at another boat named the Sea Oak, Tonga officials told U.S. prosecutors.
One witness reported seeing Fletcher grab his wife’s head while she was in the dinghy, “slam his knee into her neck and punch her in the head,” the documents said.
The next day, Fletcher went to police to report his wife had died when she slipped and fell down stairs on their yacht.
Fletcher allegedly told an acquaintance that his wife had a nerve disease and had been drunk. The person reported seeing a blood-stained bed sheet in the dinghy, and another person said Fletcher dropped the sheet into the sea, the documents said.
While in police custody, Fletcher asked a detective for permission to use the toilet, then ran out of a cell in a police station and was caught after a brief foot chase, authorities said.
The records say officers could not catch him when he fled a police station cell again in September 2016 and was last seen sailing away in a boat.
He traveled about 300 miles north to American Samoa, where he was arrested. U.S. marshals escorted him to Honolulu because American Samoa does not have a federal court.
Tonga Acting Attorney General Aminiasi Kefu told The Associated Press on Thursday the decision to free Fletcher was a disappointment.
He said he told U.S. officials it would be very unlikely that the death penalty would have been imposed. No one in Tonga receives free legal representation, and the case against Fletcher was solid, he said.
“We have very strong circumstantial evidence,” he said.
The charges against Fletcher remain active, and the country could seek Fletcher’s extradition if he travels to other countries, Kefu said. “We believe he’s committed a crime here in Tonga, and we won’t stop until we’re able to bring him to justice,” he said.
Yamaga declined to comment on the Tonga charges and said she had raised concerns with the State Department based on humanitarian issues, she said.
Allen S. Weiner, the director of the Stanford Program in International and Comparative Law, called the case “a little unusual” because the U.S. has an extradition treaty with Tonga.
Countries usually avoid signing extradition treaties when they have doubts about legal systems, he said.
Fletcher’s case is also unusual because it is usually other countries that do not want to extradite their citizens to the United States when the death penalty is a possibility, Weiner said.