"That's when he observed several victims inside then engaged the suspect. And that's when he shot him."
It was not immediately clear if the shooter knew the victims.
Two other people at the home were shot and have been hospitalized; their conditions were not known.
The identities of the victims or the shooter have not been released.
No officers were injured in the incident in Plano, which is a suburb north of Dallas.
"I've been here all my life," Tilley told the Dallas Morning News. "I've never heard of anything like this."
Authorities said all of the victims were adults. A motive for the shooting has not been released, as police are still investigating.
The shooting reportedly happened during a Dallas Cowboys watch party.
Witness Crystal Sugg, who works nearby the scene, described what she witnessed outside the home before the shooting started to Allison Harris, a reporter with Fox 4.
Sugg said she saw a man arguing with a woman outside of the home and said you couldn't really hear what the disagreement was about.
"And as they were arguing, the woman was trying to go back in," Sugg said.
"As she was going back in the house, you see the man pull out his gun and started releasing.
"So as he released, I ran back in because I didn't want to get hit by nothing."
Steven Featherland said he heard between 30 to 40 rounds go off at the scene of the shooting.
"I pulled up and cops were already positioning themselves," Featherland told the Fox4 News reporter.
"I heard you know the first couple of shots and we came outside. Next thing you know all you heard was pop, pop, pop, pop... there was like 30, 40 rounds. That's when we both went inside."
It is not clear if those shots also include the bullets fired by responding police officers.
Another neighbor, Lauryn Nichols, is shocked by the incident and said she had a friend who was in the house.
Nichols said she's not sure if her friend made it out before the shooting or not.
"I'm worried," Nichols told the newspaper. "It's like one big family here in Plano."
More than 30 police officers were at the scene Sunday night continuing to investigate, the Dallas Morning News reported.
Police were expected to be at the home throughout the night, the newspaper reports.
The Texas Rangers were responding to the scene to lead the investigation because a Plano officer was involved, according to the newspaper.
