Story highlights A Mexican official says he doesn't think Mexican helicopters crossed into the U.S.

Mexican authorities didn't fire any shots, but "criminals" were shooting, he adds

A U.S. official had said a Mexican helicopter crossed about 100 yards into Arizona

Two shots were fired near U.S. border agents, but no one was hurt, the official adds

Shots were fired this week near the U.S. border with Mexico, with a U.S. official saying they came from a Mexican law enforcement helicopter, something that a Mexican official denies.

The Mexican helicopter crossed into Arizona and fired two shots near U.S. border agents, a U.S. law enforcement official said Friday. No one returned fire, and the helicopter returned to Mexico.

No one was hurt in the incident, which happened about 6 a.m. local time Thursday about 100 yards into U.S. territory, the official said on condition of anonymity.

A separate official, U.S. Border Patrol spokeswoman Vivian McLoughlin, confirmed the helicopter crossed into U.S. territory, saying it happened at 5:45 a.m. Thursday. She did not provide further details.

Tomas Zeron, director of the Mexican attorney general's investigative office, offered a different story Friday.

While acknowledging Mexican authorities were conducting an operation "on the border," Zeron said, "I do not think we crossed the border because we brought our navigation. But it was just 100 meters from the border."

He said helicopters from the attorney general's office and defense ministry "were shot at by criminals," several of whom were later apprehended by Mexican authorities.

"The only ones doing the shooting were those that we have now detained," Zeron said.

The first U.S. source said that Mexican authorities have called U.S. authorities and acknowledged a mistake, saying shots were fired from the helicopter after it accidentally crossed the border.

But Zeron gave no indication that any mistake was made or apology was necessary, suggesting that Mexican authorities did nothing wrong.

Mexican authorities said Thursday's drug smuggling and trafficking operation culminated in the rescue of 39 migrants, detention of four people and securing of a ranch on the U.S. border.

This isn't the first case of the two allies having issues along their shared border.

Some of these have to do with efforts to corral the drug or weapons trade, with the latter being especially significant in Mexico given fears that such arms could end up in the hands of powerful drug cartels.

Citing a response he said came from the Department of Homeland Security, Rep. Duncan Hunter said earlier this month that there have been some 300 documented incursions by Mexican military and law enforcement authorities since 2004.

"While the number of unauthorized incursions by Mexican authorities is relatively few, it is imperative for our officer safety to handle each situation assertively but with sensitivity and professionalism," DHS said in a response to Duncan's request, according to the Republican congressman from California.