* Boston bombing, ricin threat, Texas fire raise anxiety
* Investigators seeking two men identified on video of bomb scene
* Hundreds turn out for Boston interfaith service (Adds quotes from service and details of investigation)
By Tim McLaughlin and Mark Felsenthal
BOSTON, April 18 (Reuters) - "You will run again," President Barack Obama told an interfaith service on Thursday for the victims of the Boston Marathon bombing, in a stirring speech aimed at bringing solace to the city and settling the nerves of a rattled nation.
At a Boston cathedral about a mile (1.6 km) from the spot where two bombs on Monday ripped through the crowds at the marathon finish line, killing three people and injuring 176, Obama sought to convey strength by vowing "we will find you" to the person or people behind the attack.
Monday's bombing began a week of security scares that rattled the United States and evoked memories of the Sept. 11, 2001 hijacked plane attacks, ranging from false bomb reports to mail sent to the White House and other federal officials containing the deadly poison ricin.
Investigators in the Texas town of West were looking into the cause of an explosion on Wednesday night at a fertilizer plant that killed up to 15 people and destroyed dozens of homes.
Some of the victims of the Boston attack suffered gruesome injuries, and at least 10 lost limbs as a result of the blasts. Investigators believe the bombs were made of pressure cookers packed with shrapnel.
"As you begin this long journey of recovery, your city is with you, your commonwealth is with you, your country is with you," Obama said. "We will all be with you as you learn to stand and walk and, yes, run again. Of that, I have no doubt. You will run again."
Hundreds of people crowded outside the Cathedral of the Holy Cross in Boston's South End. Police were out in force, and some officers listened to Obama's speech over the radio while standing next to their squad cars.
Among them was Philip Beauregard of Boston, who said, "The president was fantastic. He made it clear that the country is behind the city of Boston."
After his speech, Obama met with volunteers and Boston Marathon organizers, many of whom cared for the injured, and with victims at Massachusetts General Hospital.
'WE WILL FIND YOU'
While investigators have made no arrests yet, Obama said of the perpetrator or perpetrators of the attack, "We will find you and you will face justice."
Investigators are combing through thousands of pieces of evidence, from cell phone pictures submitted by spectators to shards of shrapnel pulled from the legs of victims.
They have not identified any suspects but they want to talk to two men who they have identified in images taken before the blast, law enforcement and national security officials said on Thursday.
"There is some video that has raised the question of those that the FBI would like to speak with," said Homeland Security Secretary Janet Napolitano said at a hearing in Congress on Thursday. "I wouldn't characterize them as suspects under the technical term. But we do need the public's help in locating these individuals."
Police had considered making an appeal to the public for more information at a news conference on Wednesday, a U.S. government source said, but the FBI canceled it after a number of delays. The FBI said on Thursday it will issue new information on the case at a 5 p.m. ET (2100 GMT) briefing.
The bombs in Boston killed an 8-year-old boy, Martin Richard; a 29-year-old woman, Krystle Campbell; and a Boston University graduate student and Chinese citizen, Lu Lingzi.
Before his visit, Obama declared a state of emergency in Massachusetts, a move that makes federal funding available to the state as it copes with the aftermath of the bombing.
Boston Mayor Tom Menino, Massachusetts Governor Deval Patrick and Cardinal Sean O'Malley also spoke at the service. Former Massachusetts Governor and 2012 Republican presidential candidate Mitt Romney also attended.
"This is Boston, a city with courage, compassion and strength that knows no bounds," said Menino, who was rolled to the podium in a wheelchair but stood for his remarks despite breaking a leg over the weekend. "We love the brave ones who felt the blast and still raced through the smoke with ringing in his ears ... to answer cries of those in need." (Additional reporting by Svea Herbst-Bayliss and Daniel Lovering in Boston, Deborah Charles, Mark Hosenball and Roberta Rampton in Washington; Writing by Scott Malone; Editing by Lisa Von Ahn and Grant McCool)