Last weekend's massacre of at least 15 people at a teen party in Ciudad Juarez was horrifying enough.

Then the authorities got involved.

Mexican officials have issued sketchy and conflicting information, including a death toll that went down. They initially said they were at a loss to explain why gunmen would open fire on decent kids in a private home. Then they produced a suspect who said the attack was part of a feud between drug-trafficking gangs, suggesting that at least some of those targeted weren't so decent after all.

When family members exploded with indignation, authorities backpedaled.

To understand why so many Mexicans lack trust in their country's law enforcement system, the brutal killings -- and the clumsy response by authorities -- offer a handy Exhibit A.

The Juarez case has already become a political hot potato, with parents of the victims calling for the resignation of President Felipe Calderon, and politicians across the spectrum jabbing fingers at one another amid the early phases of an election season.

The Senate said it would summon top officials to explain the Calderon administration's public safety strategy and the latest Juarez violence. There have been calls for a state of emergency in Juarez and even a plea for U.N. peacekeepers.

The loudest voices have been those of fed-up Juarez residents who say the authorities have added insult to their grief.

"I can assure you my nephews were good kids, students. My nephews weren't into drugs, they didn't belong to any gang," Patricia Davila, the aunt of two victims, said by telephone from Juarez.

"We're angry with the authorities because so far we don't see any solution to what happened. On the contrary, now they're staining the image of these kids by saying they're mixed up in gangs. It makes a mockery of the pain of the parents, of us relatives."

Even by the gruesome gauge of violence that has come to define Juarez, the slayings seemed to hit new extremes.

Gunmen packing high-powered weapons stormed a home around midnight Saturday and sprayed bullets, leaving bodies strewn across a blood-washed scene that in photographs resembled a slaughterhouse floor.

At least 15 people were dead, most of them younger than 20. (Authorities in the state of Chihuahua now say 15, after giving an earlier figure of 16. Local news reports say the higher figure is right.)

The incident has unleashed an unusually sharp outcry in a country that has weathered more than 16,000 drug-related killings since December 2006, when Calderon declared war on drug cartels.

Juarez has experienced the worst of it, with more than 3,700 people killed amid fighting between two cartels. The presence of nearly 10,000 Mexican soldiers and federal police officers failed to slow the bloodshed.

Parents of last weekend's victims, who say their children had no ties to crime, have vented their fury at Calderon and at Chihuahua officials overseeing the investigation.

Calderon enraged many Tuesday when he said that the incident "probably" was the result of rivalries between criminal gangs. The president, during a visit to Japan, also called the slayings "cowardly" and promised new steps to bring down violence in the border city. But the damage was done.

Chihuahua officials threw more fuel on the fire Tuesday night when they announced the arrest of a man who claimed to have served as a lookout for the shooters. He was displayed to reporters in Juarez and questioned in front of them.

The suspect, Jose Dolores Arroyo Chavarria, said the Juarez-based cartel targeted the party after getting word that members of a gang tied to its chief rival, from the northwestern state of Sinaloa, were there.

Critics accused federal and state officials of trying to wash their hands of the case by chalking it up to more drug-gang feuding. Many people see the Arroyo arrest as a sham or call him a scapegoat, according to a poll published Thursday in the Milenio newspaper.

Officials are scrambling to contain the uproar.

Chihuahua Atty. Gen. Patricia Gonzalez on Wednesday declared the victims innocent, saying they must have been slain by mistake.

Calderon said Thursday that his new steps in Juarez would seek to repair the "social fabric" through programs to improve conditions for young people.

Chihuahua Gov. Jose Reyes Baeza Terrazas, who belongs to the party that is the main rival of Calderon's, promised more arrests. Investigators said they were able to produce composite drawings of six suspects.

The governor called on residents to join hands with authorities to retake "every street corner, every neighborhood."

But such appeals are meeting with dwindling confidence among Juarez's violence-weary denizens.

"We are tired of them promising things that they're going to do, that they're going to solve cases, and then two or three weeks go by and they forget about it," said Davila, the aunt. "We're fed up. A lot of innocent people have died already, and more continue falling."

ken.ellingwood@latimes.com

*Cecilia Sanchez of The Times' Mexico City Bureau contributed to this report.