The suspect, named by police as Genesse Ivonne Moreno, 36, was with her seven-year-old son, who was critically injured in the Houston shootout.
Investigators said the attacker had “Palestine” written on the butt of her rifle and they had uncovered antisemitic writings.
But they said the motive is unclear.
A 57-year-old man was shot in the hip during the incident and has since been discharged from hospital.
The gunwoman’s unnamed son was shot in the head during the shootout and is currently “fighting for his life”, said police. It is not clear who shot the boy.
The church was in the midst of a live stream service when the first gunshots rang out. Pastor Jorge Basave at first continued on with the recording, telling the Houston Chronicle he wanted to project calm.
“I was worried my fear would be contagious,” he said.
Police said the suspect used an AR-15 rifle for the attack. She also had a .22 calibre rifle, but did not fire that weapon.
Officials told Monday’s news conference the attacker has a history of mental health issues and was placed under an emergency detention order in 2016.
Public records indicate Moreno also has a long record of arrests and convictions on assault, drug, and weapons charges.
She previously went by several other aliases, including Jeffery Escalante, and is variously listed as male and female in official records. Police described her in the news conference as a Hispanic female.
Police said they believe she had a familial dispute with her ex-husband’s family, some of whom are Jewish.
The site of the shooting was Lakewood Church, one of the largest congregations in the country, run by the renowned televangelist pastor Joel Osteen.
Moreno’s connection to the church is still being investigated, but the KHOU local television station reports her mother attended Lakewood.
The outlet added that staff at the church may have been questioned during Moreno’s contentious divorce and child custody proceedings, Montgomery County court records indicate.
Police said the suspect pulled up to the west side of the building in a white vehicle at 13:53 local time (19:53 GMT) just as a Spanish-speaking service was about to begin.
She displayed her weapon to an unarmed security guard, who proceeded to let her inside, and she then opened fire inside the church’s hallway at about 13:55.
A 28-year-old Houston police officer and a 38-year-old agent with the Texas Alcoholic Beverage Commission (TABC), both of whom were off-duty and had been hired as church security, returned fire.
Multiple rounds were fired in the ensuing gun battle, officials said, before the attacker was “neutralised” and pronounced dead at 14:07.
Houston Police Chief Troy Finner said Moreno was seen spraying “some type of substance on the ground”, but investigators have determined it posed no risk.
After being shot, she told the two officers there was a bomb, but a search by law enforcement found no explosives in her backpack or vehicle.
TABC chairman Kevin Lilly identified his agency’s officer as Adrian Herrera.
Praising the duo for holding their ground in the face of gunfire, he said: “What happened yesterday was the personification of heroism and valour.
“They were a wall that existed between worshippers and terror. Between freedom of religion and murder.”
Who is Joel Osteen and what is Lakewood Church?
Mr Osteen, a 60-year-old televangelist, took the helm of one of the most popular megachurches in the country after his father and Lakewood’s founding pastor died.
The 16,000-seat church used to be home to the Houston Rockets, an NBA team, before undergoing renovations in the early 2000s.
Under Mr Osteen’s leadership, the church has grown in size and reputation, with 45,000 parishioners attending weekly services in person, in addition to thousands who watch online and on television.
It is the third-largest church in the country, according to the Hartford Institute for Religion Research.
“Our community is devastated by today’s events and grateful for the swift actions of law enforcement,” Mr Osteen said in a statement.
“In the face of such darkness, we must hold onto our faith and remember evil will not prevail.”
Known for promoting what is called the prosperity gospel, Mr Osteen often preaches that financial blessings are gifts from God and has published many books promising that faith will increase one’s wealth.