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Arkansas native Jessy Pacheco, 30, went missing in the city of Guadalajara last week after he celebrated his graduation from the Universidad Autonoma de Guadalajara, his family told ABC News.
He attended the university's medical school for two and a half years but finished his residency in Arkansas, returning to Mexico to retrieve his diploma, said Francine Solis, his cousin's wife.
On Saturday night, Pacheco went out with a group of friends, Francine Solis said. Authorities believe Pacheco's disappearance and his friend's murder occurred in the Providencia neighborhood of Guadalajara on Saturday, according to a statement from the Jalisco State Prosecutor's Office.
Most of the group ended up leaving early, but Pacheco and another friend stayed a little longer to wait for an Uber they had called, Francis Solis said. They were accompanied by women at the time, but authorities do not have any details about their identities, Jalisco State Attorney General Gerardo Octavio Solis Gomez said in a news conference Thursday afternoon.
Pacheco's friend, identified by authorities as Carlos Alejandro Delgadillo Romero, was approached by a group of people as he left a club who beat him and shot him several times with an AK-47, killing him, Solis Gomez said.
Delgadillo Romero was found shot in the head a few blocks from the club, his family told ABC affiliate KHBS in Fort Smith, Arkansas. The 26-year-old was a California native and also a student of the university, Mexican newspaper Milenio reported.
There is no evidence that a fight occurred at the club where Pacheco and his friends celebrated, Solis Gomez said.
After the attack occurred, nearby businesses began to kick their customers out and close their doors in an effort to avoid providing any information and to not get involved, Solis Gomez said. When authorities arrived, they only found Delgadillo Romero's dead body, he added.
A report was filed later that day for Pacheco's disappearance, Delgadillo Romero said.
Pacheco's family believes he may have been abducted and have a set up a GoFundMe campaign to aid in their expenses while they stay in Mexico until he is found. He was set to return to the U.S. on Tuesday, Francine Solis said.
Pacheco planned to return home to Van Buren, Arkansas, to practice medicine after graduation, according to the GoFundMe. His mother and other family had traveled to Guadalajara to watch him graduate, Francine Solis said.
On Friday, Pacheco posted photos to Facebook of himself posing with family in his cap and gown, writing that his "dream of becoming a doctor was fulfilled today!!!"
Pachecho's cousin, Jeff Solis, described him to KHBS as an "amazing guy."
"He's a person that gets along with everybody and he's always there for everybody," Jeff Solis said. "I'm very proud of him to achieve such a goal and to hear this news is so devastating to me."
Mexican authorities have met with staff at the American consulate to keep them informed during the investigation, according to Jalisco State prosecutors.
The U.S. Department of State said in a statement that it is aware of the reports of a missing U.S. citizen.
"The welfare and safety of U.S. citizens abroad is one of the highest priorities of the Department of State," the statement read. "We stand ready to provide appropriate assistance to U.S. citizens in need and to their families."
A spokeswoman for Uber said she did not have any information on the case.
ABC News' Kate Hodgson, Conor Finnegan, Ben Gittleson and Darren Reynolds contributed to this report.