Federal prosecutors on Tuesday released transcripts of video recordings in which they say the gunman who carried out last month's fatal mass shooting at Brown University and later took his own life had admitted to planning the attack months in advance.

The four videos recorded by the suspect, Claudio Neves Valente, were discovered during a search of the storage locker in Salem, New Hampshire, where he was found dead from a self-inflicted gunshot on December 18, ending a six-day manhunt, the prosecutors said.

Valente, 48, a Portuguese national who had attended Brown two decades ago as a doctoral student in physics, slipped into an engineering building on the Ivy League campus on December 13 and opened fire with a handgun, killing two students and injuring nine others, according to police.

Authorities later determined that after fleeing the Providence, Rhode Island, scene of the Brown attack, Valente killed a physics professor from the Massachusetts Institute of Technology in a separate shooting at his home outside Boston.

Valente and the slain MIT professor Nuno Loureiro had once been classmates in Lisbon, authorities said after linking the two shooting incidents. But investigators have yet to offer a motive for either case.

Planning the Brown University shooting for a long time

The newly released transcripts of the videos, recovered by the FBI from an electronic device and translated from Portuguese to English, mark the first statements attributed to Valente since his death but shed little new light on the Brown tragedy.

According to the US Attorney's Office in Boston, which has overseen the federal investigation of the shootings, Valente "admitted that he had been planning the Brown University shooting for a long time."

In the rambling, disjointed recordings prosecutors say he made while holed up in the storage unit after the shootings, Valente refers to a shell casing injuring one of his eyes and adds that he had "already planned this for a little more" than "six semesters."

Except for repeated mentions of his eye injury, however, Valente speaks elliptically of what has happened, never explicitly talks about firing a gun or killing anyone, and offers no insights into what precipitated the violence ascribed to him.

"I don't know if there are any kind of implications of what I wanted to do or not," he said. "It was all incompetent, but at least something was done. ... The only objective was to [pause] leave more or less on my own terms and -- and it's -- it's already long overdue."

He alludes to vague, unspecified grievances, while expressing no remorse.

"To say that I was extraordinarily satisfied, no, but I also don't regret what I did," he said in one video. "I am not going to apologize, because during my lifetime no one sincerely apologized to me."

Federal prosecutors said that the evidence collected to date continues to rule out any basis for concerns about ongoing public safety threats associated with the shootings, but that investigators' search for a motive would continue.