* Gunmen kill 15 people at student party
* Mexican president to call for broader strategy
* Senate wants to hear from top security officials
(Updates with suspect's testimony, army quote)
By Julian Cardona
CIUDAD JUAREZ, Mexico, Feb 2 (Reuters) - Angry families of 15 people slain by gunmen during a high-school party near the U.S. border blamed Mexican President Felipe Calderon on Tuesday for failing to prevent the mass killing.
The Senate also demanded the government explain how the 13 teenagers and two adults could be gunned down at the birthday party in Ciudad Juarez despite a heavy army presence in the city aimed at quelling rampant drug violence.
"Until we find who is responsible, you Mr. President are the assassin," read a banner scrawled with black ink as dozens of angry relatives, some dressed in black, gathered outside the bloodstained house where the massacre took place in the early hours of Sunday.
Families lit candles and laid flowers by photos of the slain youngsters that they placed by the house and left messages saying "we're going to miss you boys, we love you."
Officials presented to the media a 30-year-old man suspected of involvement in the attack.
Oscar Arroyo was questioned by police with his back to reporters. He said he had been paid by the powerful Juarez cartel to watch for police while hitmen attacked the party.
Arroyo said he overheard from a cartel leader called Diego that the Juarez cartel believed those at the party were rival traffickers working for Mexico's top drug kingpin, Joaquin "Shorty" Guzman, who is vying for control of Ciudad Juarez.
"They said that those at the party were all Shorty's people," Arroyo told police at the news conference.
Parents of the slain teenagers say the students had nothing to do with drug trafficking and Mexican officials are divided over a possible motive.
Both the army and Patricia Gonzalez, attorney general for Chihuahua, have said the massacre could be linked to drug cartels, but Ciudad Juarez Mayor Jose Reyes Ferriz said the shooting was random.
Calderon, who sent thousands of troops there last year to clamp down on killings, vowed to respond.
"The federal government will analyze extending and strengthening its strategy in Ciudad Juarez," he said during an official visit to Japan. He signaled a bigger emphasis on social issues and not just a crackdown on crime.
Federal police and soldiers set up checkpoints across Ciudad Juarez on Tuesday and military helicopters flew overhead, but similar shows of force have failed to have an impact over the past year.
'LEAVE THOSE KIDS'
Many Mexicans are losing patience with Calderon.
Since he deployed the army across Mexico in late 2006, more than 17,000 people have been killed in drug gang violence that has only escalated as the army crackdown has fueled turf wars between rival cartels. The bloodshed is a worry for the U.S. government, foreign investors and tourists alike.
"What would President Calderon do without his bodyguards? He wouldn't last one day alive," said a female cousin of two teenagers killed in the shooting, her face reddening with anger.
Witnesses say hitmen jumped out of sport utility vehicles and sprayed bullets at the teenagers celebrating a classmate's birthday just across the border from El Paso, Texas.
The army said 15 people died in Sunday's attack, although some Mexican media put the death toll at 18.
Neighbors in the working class district where the attack happened said some residents ran into the street as they saw the armed men going into the party just after midnight, shouting "leave those kids, they haven't done anything wrong."
The gunmen opened fire at three houses in the street, finally storming into the birthday party just as the youngsters were lighting candles on the cake, survivors said.
Bodies lay on the street and blood ran out of the house.
Ciudad Juarez has become the epicenter of Mexico's drug war, as gangs fight over smuggling routes into the United States and access to local addicts. Law and order is breaking down in the city, with gunmen killing rivals and innocent civilians at will.
Some 2,650 people died in drug violence in Ciudad Juarez last year. Murders have jumped since the start of 2010.
Mexico is the key transit route for U.S.-bound cocaine from South America and a top producer of marijuana and heroin. ($1=12.86 pesos) (Additional reporting by Miguel Angel Gutierrez in Mexico City; editing by Chris Wilson)