An undisclosed number of arrests have been made relating to last week’s massacre of three women and six children at the hands of suspected cartel gunmen in northern Mexico, a Mexican security official said Monday.

The new arrests, which come as Mexican authorities appeared to struggle in pinpointing the cartel responsible for the heinous attack, seemed to leave some officials bewildered over the development.

FBI TO JOIN MEXICO CARTEL AMBUSH INVESTIGATION INTO KILLINGS OF MOTHERS, CHILDREN

Speaking to reporters in Mexico City, Security Secretary Alfonso Durazo said federal prosecutors, as well as those from Sonora, the embattled state in northern Mexico where the families were slaughtered, were in charge of the investigation.

He didn’t specify the number of arrests made or give any information on what organization they belong to, telling reporters “it’s not up to us to give information.”

However, when Reuters reached out to the government of Sonora for information about the arrests, a spokeswoman replied: “We don’t have that information.”

MEXICAN MASSACRE SUSPECT ARRESTED AT BORDER NOT TIED TO ATTACK; CONFUSION OVER WHICH CARTEL WAS RESPONSIBLE

This is the second time the Mexican government has announced an arrest in the case.

The first suspect, arrested near the U.S.-Mexico border a day after the killings, was determined not to be linked to the attack, despite being found with four assault rifles and two hostages bound and gagged in his bulletproof vehicle, Durazo said last week.

Authorities have faced mounting pressure to bring those who carried out the killings to justice since last week’s attack. On Sunday, the Mexican government invited the FBI to join the investigation, nearly a week after the bureau first offered its help.

The families, who lived in the community of La Mora, a decades-old settlement part of an off-shoot of The Church of Jesus Christ of Latter-day Saints in northern Mexico, were traveling on a remote dirt road in broad daylight when gunmen sprayed their three SUVs with bullets. Photos of the grisly scene showed a burned-out SUV riddled with bullet holes and a blood-stained car seat.

While it’s unclear which groups are the focus of the investigation, Mexican authorities have mentioned that several cartels operate in the area, including La Linea, Los Jaguares and Jalisco Nueva Generacion. La Linea, the armed wing of the Juarez drug cartel, was initially believed to be involved.

MEXICO CARTEL AMBUSH: AMERICAN FATHER WHOSE RELATIVES WERE KILLED SAYS HE'S LEAVING THE COUNTRY

However, officials have not confirmed the involvement of any of the groups.

The Mexican government has suggested the families were not the target of the attack, saying the gunmen may have mistaken the families’ large SUVs for those of a rival gang. Officials have noted the ongoing, violent turf war between cartels in the region.

The killings sparked outrage in the U.S., with President Trump calling on Mexico to wage “war” on the drug cartels following the attack.

CLICK HERE FOR THE FOX NEWS APP

But Mexico's President Andrés Manuel López Obrador, who has faced mounting criticism for his government's "hugs, not bullets" approach to cartel violence, maintained that war was not the appropriate response to the growing deaths at the hands of cartels.

Fox News' Lucia I. Suarez Sang and the Associated Press contributed to this report.