The Department of Homeland Security said its inspector general would investigate the fatal shooting of a Colombian immigrant by an agent last month in Maine, but scant information about the effort has frustrated the victim’s family.

The man, Johan Sebastián Durán Guerrero, was killed by an immigration agent during a July 13 confrontation in the coastal city of Biddeford. Neither the D.H.S. nor Immigration and Customs Enforcement has said publicly whether Mr. Durán Guerrero, 25, was the target of the enforcement operation. But the D.H.S. defended the officer who opened fire, saying he had done so while “fearing for public safety.”

In an effort to draw attention to the many unanswered questions, Democratic members of Congress held a public forum on Monday in Biddeford and were joined by Mr. Durán Guerrero’s relatives in demanding more information.

The auditorium where the forum was held was packed, with about 200 people, many of them wearing T-shirts and carrying placards critical of ICE. The crowd punctuated the lawmakers’ comments with raucous applause, and occasional chants of “ICE out,” and “abolish ICE.”

Representative Chellie Pingree, a Democrat of Maine, who organized the forum, said that the lack of video footage was concerning.

“If there were active body cameras on every law enforcement member who was participating there, we wouldn’t be looking at some scratchy videos from one store that’s over here, or one person that’s standing in their doorway that’s trying to get it,” Ms. Pingree said at a news conference after the hearing. “We would actually know every moment that’s happened at the scene.”

The shooting in Maine came less than a week after a fatal immigration encounter in Houston, and the two deaths prompted a fresh wave of attention on aggressive enforcement tactics by federal agents carrying out President Trump’s mass deportation campaign.

Ben Gideon, a lawyer representing the family of Mr. Durán Guerrero, said at the news conference that the man’s partner, Karolina Rojas, has been interviewed by state investigators but had not been contacted by any of the federal agencies involved.

“Any reasonable investigation would include interviewing the family and eyewitnesses,” Mr. Gideon said.

The D.H.S. had said that agents were monitoring what they believed to be the home of someone who was in the country illegally and had been issued a removal order.

The D.H.S. and the Department of Justice declined to answer direct questions about whether the killing was being investigated. The Justice Department referred the question to the D.H.S., which said that its inspector general would investigate the discharge of the agent’s firearm but said nothing about investigating Mr. Durán Guerrero’s death.

Immigration agents have shot at more than 20 people during the Trump administration’s immigration crackdown, including many who were in vehicles, some of them fatally.

Mr. Durán Guerrero had a pending asylum application and a valid work permit, and no removal order, Ms. Pingree said.

Mr. Durán Guerrero had worked as a food delivery driver and a cleaner at a veterinary clinic. He and his partner had a 3-year-old daughter.

Gov. Janet Mills of Maine said at the hearing that she was heartbroken by the shooting but that she was confident that the state attorney general, Aaron Frey, will conduct a thorough investigation.

Mayor Liam LaFountain of Biddeford also addressed the officials. He said that Biddeford has been devastated by the shooting and that there has been no federal support to deal with the aftermath.

“The authority was federal,” Mr. LaFountain said. “The burden has been local.”

Omar Durán, Mr. Durán Guerrero’s father, said he feels “a deep pain” when Mr. Durán Guerrero’s daughter asks why her father doesn’t come home from work.

“The death of Johan Sebastián, under those conditions, at the hands of ICE, fills our community with fear and uncertainty,” he said. “That is why I ask only of God and of the justice system that my son’s case be brought to light, along with the case of the many others who have had to go through something like this.”

Alan Feuer contributed reporting.