The sting operation started last month with undercover officers buying heroin for $40 a pop from a three-family home in Roslindale, a prosecutor said Wednesday.

Three different times, Eric Rosado-Gonzalez of Dorchester allegedly met with undercover police at 37 Newburg St. and sold them drugs, Suffolk Assistant District Attorney Mark Hallal said.

Then during a drug deal on Sept. 28, Rosado-Gonzalez, 25, allegedly made another offer: He would sell them firearms.

The proposal, Hallal said, led two undercover State Police troopers to Newburg Street Tuesday afternoon, where they planned to purchase two handguns and an Uzi submachine gun from Rosado-Gonzalez for $1,800.

But when troopers arrived around 4:30 p.m., Rosado-Gonzalez and another man, Luis Colon, 25, tried to rob the undercover officers, instigating a gunfight in a residential neighborhood followed by a manhunt, officials said. No one was hit by gunfire, authorities have said.

“These defendants allegedly set in motion events that could have gotten someone shot or much, much worse,’’ Suffolk District Attorney Daniel F. Conley said in a statement. “We’re extremely lucky that no one was seriously hurt.’’

At their arraignment Wednesday in West Roxbury Municipal Court, Hallal said Rosado-Gonzalez and Colon simultaneously pulled guns on the undercover troopers.

Rosado-Gonzalez climbed into an unmarked vehicle driven by one undercover trooper, held a firearm to his head, and threatened to kill him, Hallal said.

“The undercover officer in the car was able to push Rosado-Gonzalez out of the car and escape,’’ he said. “Rosado-Gonzalez got out of the car and fled from the area.’’

Around the same time, Colon was on the porch of 37 Newburg St. with another undercover trooper involved in the operation, Hallal said.

“During the course of that struggle, Luis Colon removed a firearm from his waistband and fired at the officer multiple times without striking him,’’ he said. “Other undercover officers on scene then exchanged gunfire with Luis Colon. He was not hit.’’

Colon, a recent transplant to Boston from Lawrence, was arrested a short time later on nearby Colberg Avenue, Hallal said. He said a firearm was located “within feet’’ of Colon.

Rosado-Gonzalez, a meat cutter at America’s Food Basket in Mattapan, was arrested at 9:15 p.m. Tuesday at the intersection of South and Robert streets, a police report said.

He later told investigators that he and Colon had planned to rob the officers, Hallal said. Attorney Paul V. Marino, who represents Rosado-Gonzalez, said he’s seen no evidence that his client made that admission. Rosado-Gonzalez believes police arrested the wrong man, he said.

“He denies the allegation,’’ Marino said.

He and attorney Steven M. Key, who represents Colon, said they have not seen evidence that police seized firearms from the men.

“I have to question it,’’ Key said. “Nowhere in the narrative did they say [they] recovered a firearm.’’

Rosado-Gonzalez is charged with assault with a dangerous weapon, armed assault with intent to rob, and assault and battery with a dangerous weapon, the district attorney’s office said. Colon is charged with several firearms offenses and armed assault with intent to murder.

Judge Kathleen Coffey set bail at $500,000 each and revoked their bail on pending cases in different courts. Their next court appearance is planned for Oct. 28.

State Police and Boston police worked together on the investigation, officials said. David Procopio, a State Police spokesman, said Boston is safer because the men were apprehended.

“What happened [Tuesday] night underscores the risks undercover troopers and officers accept on a regular basis to get guns and drugs off the street,’’ he said.

At the home where the gunfire erupted, bullet holes pierced a window and the siding.

A bullet was also lodged into the exterior of a neighboring residence. Gino Johnston said his 11-year-old son, Patrick, was on the first floor when the gunfight started.

“He got on the floor, and he got his way to the basement,’’ Johnston said.

The boy hid under a table for about 30 minutes until police rescued him, said Johnston, a painter who was working on nearby Beech Street when the shooting started.

“They did a great job, and they put him right in his mother’s arms,’’ Johnston said. “He was visibly shaken. I know my son. He was holding back the tears.’’

Kim Augusta, who lives nearby, said she heard gunfire, then someone yelling, “Get down here now!’’ The yelling was followed by more gunfire and a screeching sound, she said.

“I thought it was fireworks or something. It never occurred to me that it would be a shooting in front of my house," she said. “That’s just so absurd to me.’’

Laura Crimaldi can be reached at laura.crimaldi@globe.com. Follow her on Twitter @lauracrimaldi.