(CNN) -- The 15-year-old Mexican youth who was shot and killed by a Border Patrol agent had a history of involvement with human smuggling and was on a list of repeat juvenile offenders, U.S. Customs and Border Protection spokesman Mark Qualia told CNN Thursday.

The victim, Sergio Adrian Hernandez Guereca, had been apprehended by U.S. officials on more than one occassion but was never criminally charged, Qualia said.

The use of juveniles to smuggle people across the border is a common tactic for smugglers, he said.

Meanwhile, a video obtained by CNN casts doubt on the Border Patrol agent's claim that he was surrounded by rock-throwing suspected illegal immigrants when he fatally shot the boy on the border at Ciudad Juarez.

CNN obtained the video, which was shot by a witness on a cell phone camera from the Mexican side of the border, from affiliate Univision. The video aired on its program Primer Impacto late Wednesday evening.

The video shows part of the buildup before the incident, with several individuals running underneath the Puente Negro, a railroad span that connects the two countries.

Monday night's incident started around 6:30 p.m. when Border Patrol agents responded to a report of a group of suspected illegal immigrants being smuggled into the United States near the Paso del Norte port of entry, FBI Special Agent Andrea Simmons said.

In the distance, a U.S. Border Patrol officer on a bicycle can be seen making his way toward the area. Seconds later, the officer can be seen getting off the bicycle and approaching two of the four suspected Mexican nationals who had just crossed through an opening in the fence. One of the suspects is detained by the officer, but never handcuffed, and is dragged a short distance. This happened on the U.S. side of the border.

Moments later, the officer points what appears to be his firearm in the direction of a second suspect, standing about 60 feet away from the officer -- on the Mexican side of the border. The video shows the suspect running away.

Seconds later, two gunshots can be heard on the video. A third gunshot is heard in a different sequence of the tape. After the shooting, another suspect is seen running in the upper left side of screen away from the incident.

"They're throwing rocks," witnesses screaming in Spanish can be heard in the background of the video as the officer opens fire. "They hit him ... they hit him."

The video contradicts Simmons' account. She had said: "This agent, who had the second subject detained on the ground, gave verbal commands to the remaining subjects to stop and retreat. However, the subjects surrounded the agent and continued to throw rocks at him. The agent then fired his service weapon several times, striking one subject who later died."

A federal law enforcement official told CNN that the FBI's use of the word "surrounded," was "probably not the best choice of words," and that it is more accurate to say that people were nearby throwing rocks.

The FBI has been studying videos of the incident and said some of the video does show rocks being thrown at the Border Patrol agents, the official said.

Hernandez Guereca was a secondary student in Juarez.

"The young man was not armed," said Sergio Belmonte, Ciudad Juarez spokesman. "He did not have the physical size to threaten anyone. The aggression (by the U.S. agent) is evident." Belmonte said Hernandez was shot in the head.

"My people have spoken to his family. His dad says he was a straight-A student. His secondary school even sent him on an academic trip because of his good grades," Mayor Jose Reyes Ferriz said.

A memorial for Hernandez was to be held in Juarez Thursday morning, according to Reyes. Local politicians were expected to attend.

Reports that the Mexican military may have drawn weapons on U.S. Border Patrol agents, which surfaced earlier Wednesday, could not be immediately confirmed. The tape released by Univision did not show any Mexican military troops.

"We are aware of those reports, but I cannot confirm them to you at this time," said Mexican military spokesman Enrique Torres. "I plan on speaking with the individuals who are said to have been involved, but I can't and won't confirm that to you. I cannot speculate."

The Mexican government has requested a quick and transparent investigation into the fatal shooting.

Mexico "reiterates that the use of firearms to repel a rock attack represents a disproportionate use of force, particularly coming from authorities who receive specialized training on the matter," the Foreign Ministry said Tuesday in a news release.

Simmons told CNN earlier that she did not know whether the person who was shot was on the Mexican or U.S. side of the border, but the agent never left U.S. territory.

The body was found on the Mexican side of the border, Simmons said.

Peaceful protests against the U.S. Border Patrol were held in Juarez on Wednesday, near the site of the fatal shooting.

In video shot by CNN affiliate KVIA, a man holding a bullhorn is seen pacing back and forth on the dusty streets of Juarez, pleading "Justice for Hernandez" to any passerby who would listen.

The shooting comes less than two weeks after the May 31 death of a Mexican illegal immigrant who had been detained three days earlier by border agents in California.

A suspect identified as Oscar Ivan Pineda Ayala was initially detained on the Rio Grande levee, said the FBI, which is leading the investigation.

"The growing frequency of this type of event reflects a worrisome increment in the use of excessive force on the part of some border authorities," the Mexican Foreign Ministry said.

According to the ministry, the number of Mexicans who have been killed or wounded by U.S. border authorities has increased from five in 2008 to 12 in 2009 and 17 so far this year.

Earlier, Qualia, the U.S. Customs and Border Protection spokesman, said he could not comment because he does not know where the Mexican government obtained its statistics.

But Qualia said there were 799 assaults on border agents from October 1, 2009, through May 31 -- up from 745 assaults for the same time period in 2007-08 and 658 for the same span in 2008-09.

Lethal force, he said, is allowed "when an agent is in imminent threat of physical or bodily harm, which could cause death or injury or in protection of an innocent third party."

The determination of when to use lethal force, Qualia said, is made by each individual agent at the scene.

From October 1 through May 31, he said, Custom and Border Protection agents used their firearms 31 times.

Rock-throwing can be considered a dangerous assault, Qualia said: "They're not chunking pebbles."

CNN's Nick Valencia, Arthur Brice, Jeanne Meserve and Devon Sayers contributed to this report.