TEMPO.CO, San Bernardino - Relatives of a school teacher shot dead by her husband in a San Bernardino, California, classroom have told investigators the slaying capped a brief, turbulent marriage and breakup marked by accusations of infidelity and threats, police said on Tuesday.

Police also revealed that surveillance video showed the shooter initially had tried to get into the school through a locked door, forcing him to gain entry instead by way of the front office, where he signed in and was allowed free access to the classroom by staff who recognised him.

School employees, however, were apparently unaware of the marital discord believed to have led the gunman, an unemployed former pastor and maintenance worker, to fatally shoot his estranged wife, Karen Elaine Smith, and an 8-year-old student in front of their class before reloading his revolver and killing himself.

The slain student and another boy hit by gunfire happened to be just behind Smith and were believed to be unintentional victims. The second child, aged 9, though badly wounded, was expected to survive. He remained hospitalized on Tuesday.

Later on Tuesday, the Mexican government confirmed in a statement that the deceased student was a Mexican national and that it "profoundly laments" his killing.

The shooting at North Park Elementary, the latest of dozens of U.S. schools traumatized by armed intruders in recent years, reopened debate about what educators can do to safeguard students against mounting gun violence.

It was especially wrenching for San Bernardino, the "Inland Empire" town about 65 miles east of Los Angeles where a shooting rampage 15 months ago left 14 people dead and more than 20 wounded at an office holiday party.

Unlike the 2015 massacre, carried out by a radicalized Muslim couple in what authorities described as an act of terrorism, police said the latest shooting stemmed from a domestic dispute between the suspect and his wife.

BRIEF, STORMY MARRIAGE

Monday's gunman was identified as 53-year-old Cedric Anderson of nearby Riverside, who according to police had a criminal record that included four past arrests, most recently in 2013 on assault and weapons charges. But he was never convicted, Police Chief Jarrod Burguan told reporters on Tuesday.

Burguan said it was unclear whether that arrest had anything to do with Anderson's wife, also 53.

The couple had known each other for more than four years and were married in January. But Smith moved out of their home in mid-March amid domestic strife, Burguan said.

According to interviews with her family, Anderson had accused his wife of infidelity, the chief said. He said Smith also had confided to relatives that she was concerned about his "odd" behaviour and that "he had made some threats toward her."

Burguan did not elaborate except to say there was no "specific threat to shoot her."

"It appears that he had been making efforts to contact her and have her come back home, and she was resistant to that. And I don't know if that just reached the boiling point or what it was. Nor do we know exactly why he chose to do this at the school," the chief said.

Burguan said Anderson, who had shown his identification to school employees in the front office before being allowed in to her class unescorted, kept his weapon concealed until opening fire without saying a word.

Fifteen special-needs students and two adult teacher assistants were in the classroom with Smith during the attack, in which 10 shots were fired, police said.

No suicide note as such was found. But detectives turned up a handwritten letter at Anderson's home in which he wrote about "the relationship and fixing things, and it also made reference to feeling dishonored, and moving forward," Burguan said. Still, there was nothing inherently alarming about the letter's language, he said.

REUTERS