BEIRUT, Aug 14 (Reuters) - A Lebanese army investigation into a bomb attack that killed 15 people, including 10 soldiers, is focusing on militant Sunni Islamist involvement, a security source said on Thursday. The attack in the northern Lebanese city of Tripoli on Wednesday was the deadliest against the army since its 15-week battle with al Qaeda-inspired militants in the north last year. The bomb, which was planted at a bus stop, wounded 45 people.

"Some people have been detained and the investigation is focusing in the direction of extremist, fundamentalist elements," the security source told Reuters. He did not say how many people had been held or whether they had been charged.

The army lost 170 soldiers last year while putting down an insurrection by the Fatah al-Islam militant group, which was based at the Nahr al-Bared Palestinian refugee camp.

"There could be remnants of Fatah al-Islam or those cooperating with it and linked to al Qaeda," the source said.

More than 200 of the group's members, who were drawn from across the Arab world, were killed in the battle last year. Army investigations into Fatah al-Islam found links to al Qaeda.

Shaker al-Abssi, the group's Palestinian leader, was initially thought to have been killed in the fighting but a DNA test failed to identify a body which his family said was his.

A speaker identified as Abssi threatened attacks on the army in a voice recording posted on a Web site in January and a statement in his group's name claimed responsibility for a bomb attack which killed one soldier in the north in May.

"This is unfortunately going to continue," said Ahmad Moussalli, a professor at the American University of Beirut and an expert on Islamic movements. "Expect more explosions."

Lebanese and Palestinian political sources and diplomats in Beirut told Reuters in April that Sunni Islamist militants inspired by al Qaeda and possibly linked to it posed a growing threat to security in Lebanon.

Tripoli, a predominantly Sunni city, and the surrounding areas have long been a recruiting ground for Islamist groups. In 2000, 40 people were killed in a battle near Tripoli between the army and a militant group called Takfir wal Hijrah.

Scores of Lebanese and Palestinians living in Lebanon went to Iraq to join anti-U.S. insurgents, including al Qaeda.

Their return to Lebanon is seen as adding to security risks in a country which is home to large Shi'ite and Christian populations, borders Israel and hosts thousands of U.N. peacekeepers -- all possible targets for Sunni militants. The soldiers were given a military funeral on Thursday. Their coffins, draped in Lebanese flags, were carried by soldiers in a procession at a Tripoli stadium. (Writing by Tom Perry, editing by Mark Trevelyan)

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