(CNN) -- Federal authorities arrested a second person in connection with last weekend's massacre at a house party in southern Ciudad Juarez, Mexico, that killed 15 people, government officials said Saturday.
Authorities with the city's federal security operations told CNN that Israel Arzate Melendez was a co-conspirator in the killings.
In a press conference Saturday, Arzate confessed to being a hitman for "La Linea" or the Juarez cartel. He said he acted as a lookout for protection while 14 gunmen entered the party and began shooting.
"We were looking for a young guy ... who was with the Artist Assassins," Arzate said, referring to a street gang affiliated with the rival Sinaloa cartel. "El Rama [the leader of the group] told us to kill everyone, including women."
Arzate also admitted to shooting and killing one person.
Oscar Dolores Arroyo Chavarria, 30, a member of the Juarez cartel, was arrested Monday after a shootout with soldiers, officials said. The alleged leader of the massacre was killed in the shootout.
Authorities said Arzate became a suspect after Chavarria gave them a physical description and photos of Arzate. Witnesses described seeing a 2006 gray Jeep Grand Cheerokee at the shooting scene. Arzate was stopped Thursday night while driving a vehicle that matched that description.
Investigators sent CNN detailed documents mapping out the developments of their investigation, including sketches of how they believe the killings were carried out.
According to the documents, investigators said the gunmen met at a restaurant on January 31 between 7 and 8 p.m. local time. It was there where they were met by a cartel member who informed them where and when the party would be held.
One car was sent to scope out the scene, returning 15 minutes later to inform the others to arrive in their cars one by one to familiarize themselves with the site.
Documents said partygoers who tried to flee were shot in the back while gunmen could be heard saying "There they go, there they go!"
The hitmen were said to have told their victims "You, too," before being shot, according to the documents. The documents also said the gunmen first told the victims "women and children out," but were later told to execute everyone.
Also Saturday, a government spokesman told CNN that a girl found dead near the site of last weekend's massacre was killed in an isolated incident. She was initially reported to be part of the group of partygoers.
"The death toll from the party killings is 15 and has always been 15," said Enrique Torres, spokesman for the city's federal security operations.
The girl, whose identity was not released by authorities, was killed around the same time gunmen opened fire on the group celebrating a birthday party, Torres said. Most of the victims were students ranging between 13 and 19 years old.
The conflicting reports by Mexican officials, along with what residents are calling a slow response to the crime scene, heightened a pre-existing frustration within the community.
Since Sunday's shootings, relatives of the victims and community members have called for the president to resign and human rights groups have petitioned for United Nations peacekeepers to be sent to Juarez, according to local reports.
The gunmen belonged to the Juarez drug cartel and believed that their victims were members of a gang affiliated with the rival Sinaloa cartel, Torres said.
"One or two of the adults killed at the party had ties to organized crime," Municipal Police spokesman Jacinto Seguro said. "That line of investigation continues."
But no connection to organized crime has been established among the students killed in the attack, five of whom played American-style football, police said.
One of the students, Jose Adrian Encina Hernandez, had been recognized by the governor of Chihuahua for his outstanding academics, police said.