By Suleiman Al-Khalidi and Mark Hosenball

AMMAN/WASHINGTON (Reuters) - U.S. investigators are in Jordan to interrogate the Jordanian-American uncle of the gunman who attacked military offices in Tennessee last week, killing five U.S. servicemen, said the uncle's lawyer and U.S. government sources on Wednesday.

Mohammod Youssuf Abdulazeez, the suspected shooter, was killed in a gunfight with police on July 16 after he sprayed bullets at a military recruitment center in Chattanooga, Tennessee, and a nearby Naval Reserve Center.

The 24-year-old Kuwaiti-American had long-standing psychological and substance abuse issues, his family said.

Last year, to help with his recovery, he was taken to stay with his uncle, Assad Ibrahim Abdulazeez Haj Ali, in Amman, the family said.

Investigators said he remained there between April and November, under the supervision of his grandfather and the uncle. A source close to the family said the uncle was Abdulazeez's mother's brother.

Jordanian authorities arrested Haj Ali on Friday and he was being interrogated by both Jordanian and U.S. investigators, his lawyer Abdul Qader al Khatib said by telephone.

Three sources familiar with the U.S. investigation said investigators had information indicating the uncle might have had connections to the Palestinian militant group Hamas. The sources asked for anonymity when discussing sensitive information.

One source cautioned that Abdulazeez was born in Kuwait, but his family, including the uncle, are of Palestinian extraction, so contacts with Hamas might not be significant.

Investigators have evidence Abdulazeez had been searching for jihadist material that may have inspired the attack, a source told Reuters on Monday.

A Jordanian security source confirmed U.S. investigators had arrived and said they were involved in questioning Haj Ali at a high-security facility in Jordan's intelligence headquarters.

"We are gathering clues and we need to establish whether this was a sole act or one that is linked to a wider terror network," the source said.

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"At this stage of the interrogation we cannot rule out anything."

Khatib called on the Jordanian authorities to allow him to attend their interrogations, saying he feared his client could be forced to testify under duress.

"I fear he may be tortured to extract a confession," he said. He had been told he could meet with his client next Sunday, but said this was not soon enough.

Jordan regularly denies claims by human rights activists that it tortures security detainees to obtain confessions.

Khatib said there was nothing to indicate that the uncle or the nephew were militant Islamists. "They (the investigators) are focusing on whether my client had influenced (his nephew) and if he had motivated him. There is no basis to this as both do not adhere to any religious ideology," he said.

Investigators have found no evidence Abdulazeez was in direct or indirect contact with Islamic State or other organized militant groups before the shootings, U.S. officials said, though they are still digging into his background and travels.

An FBI spokesman declined to comment.

(Editing by Kevin Drawbaugh, Sylvia Westall and James Dalgleish)