Tight security greeted students Wednesday, hours after six people were shot at a bus stop in what officials believe was a high school fight about a girl.

"We've got extra officers at several high schools," said Clark County School District police Sgt. Ken Young, who said the investigation remained focused at a northeast Las Vegas high school where the dispute originated.

The six were shot Tuesday, shortly after a group of Mojave High School students got off a school bus in a working-class neighborhood. Police were searching for at least two suspects.

An 18-year-old man was in critical condition and a 17-year-old boy was upgraded from critical to serious condition, both with gunshot wounds to the torso, said Cheryl Persinger, a University Medical Center spokeswoman.

Four people, including at least two boys and a girl who are under 18, were treated for gunshot wounds to their arms and legs and were released, she said. All four are students at Mojave High School who had just stepped off the bus, which was coming from the school, police said.

Authorities would not release any other information about the two victims who were still hospitalized, including whether they were students or had been on the bus.

Police said they believed the shooting was linked to a fight at Mojave earlier in the day. The brawl had been contained by school police, who arrested three male students, officials said.

Sheriff Doug Gillespie did not describe how he believed the incidents were connected. The fight at the school did not appear to be gang or race-related, he said.

"At this point, the indicators that we have found ... it's a dispute over a girlfriend," Gillespie told reporters.

Police believe the suspects were waiting at the bus stop for the bus to arrive. Gillespie said the shooting did not appear to be random.

Investigators said they found 9 mm and .45-caliber shell casings at the scene. As officers arrived, several witnesses ran from the bus stop, police said. The suspects left on foot, Gillespie said.

Those arrested after the school fight are about 16, school district police Lt. Ken Young said.

Officials said the district activated a crisis-response plan, including counseling at Mojave.

"I think the No. 1 question on many people's minds tonight is, 'Is it safe to send my kids to school tomorrow?"' Gillespie said Tuesday. "The answer is yes. I am a father, I have a daughter and she will be attending school tomorrow."

Mojave had some 2,300 students last year, about evenly divided among blacks, whites and Hispanics, according to district materials.

The shooting occurred just before 2 p.m., a couple of blocks from two elementary schools. Both were locked down temporarily but reopened in about an hour, district spokesman Michael Rodriguez said.