* Obama calls shootings "murderous and craven"

* Questions raised about possible warning signs

By Ross Colvin

FORT HOOD, Texas, Nov 10 (Reuters) - President Barack Obama vowed on Tuesday to make sure the gunman who killed 13 people in a rampage at a U.S. Army base in Texas pays for his crimes.

Leading a memorial service for victims of an attack blamed on a Muslim Army psychiatrist, Obama reminded Americans they were enduring "trying times" while fighting wars in Iraq and Afghanistan but said there was no justification for what he called an "incomprehensible" tragedy.

"No faith justifies these murderous and craven acts. No just and loving God looks upon them with favor," Obama told a crowd of 15,000, many of them soldiers in camouflage, on a parade ground outside Fort Hood's headquarters.

"And for what he has done, we know that the killer will be met with justice -- in this world, and the next."

The shootings marked the latest blow to a U.S. military under strain from its combat duties as Obama weighs sending thousands more troops to the increasingly unpopular war in Afghanistan even as he winds down America's role in Iraq.

The somber ceremony came amid questions about whether authorities missed warning signs about the alleged gunman, Major Nidal Malik Hasan, who spent years counseling severely wounded soldiers and was soon to be deployed to Afghanistan.