Japan is demanding an explanation on why civilians were at the controls of a submarine when it smashed into fishing vessel off Hawaii.
Nine people are missing following the collision - including four teenage school pupils.
The fishing boat was owned by a high school.
On returning to Japan, Ietaka Horita, the school's head said he was torn between returning and staying in Hawaii until the missing people were found.
He said: "It was very painful for me to come back from Hawaii, leaving nine people still missing and their families.
"The top priority for me is to see that they are rescued, and I need to know who is responsible."
He added that he was "enraged" to hear that civilians were allowed at the controls of the submarine.
Japan is urging America to continue searching for those missing since the accident on February 9 - four 17-year-olds, two teachers and three crew members.
Horita added: "Even though it might eventually become clear that there's little hope, families want to actually see their dead and hug them, to help them come to terms with it."
The Ehime Shimbun newspaper said in an editorial: "Although the United States says it is doing its best in the investigation, we cannot trust that claim."