Police in Brockton executing a search warrant last week at a residence where neighbors had complained of suspected drug activity found an unattended 1-year-old boy in a tub as they went through the house and seized fentanyl, a gun, cash, and other drugs, an investigator said Tuesday.

The child found inside 31 Falconer Ave. last Friday appeared unharmed, said Brockton police Sergeant George Khoury. Authorities contacted state child welfare officials about him, he said.

Police arrested four men and recovered more than 400 grams of the synthetic painkiller fentanyl, $4,000 in cash, a loaded, .40 caliber large-capacity firearm, nine rounds of ammunition, 74 pills of the prescription medication Xanax, and other materials associated with the drug trade, officials said.

The men have been charged with trafficking fentanyl and other drug offenses, authorities said.

“Fentanyl is killing people,’’ Mayor Bill Carpenter said Tuesday. “It’s our number one target right now.’’

Investigators sought a search warrant for the single-family home after Brockton police tracked complaints from neighbors about drug deals and constant foot traffic coming to and from the residence over a period of two months, Khoury said.

Just before noon last Friday, officers watched the target of the investigation, Jesse Veiga, 24, and another man, Morgado Barros, 21, leave the home and drive to a parking lot near EbLens, a clothing retailer on Belmont Street, according to the Plymouth district attorney’s office.

While in the parking lot, investigators watched Veiga and Barros engage in what they believed to be a drug deal and took both men into custody, the DA’s office said. Police searched Veiga and found some fentanyl packaged for distribution and cash, prosecutors said.

Officers then went back to the home on Falconer Avenue, searched it, and arrested two more men. Among them was Naldo Monteiro Barbosa, 18, who faces drug charges and accusations that he illegally carried a firearm and ammunition, the DA’s office said.

The fourth man arrested was Alexandre Centeio, 23, of Medford, prosecutors said.

The men were arraigned Monday in Brockton District Court. Bail was set at $10,000 for Veiga, $100,000 for Barros, and $50,000 for Centeio, authorities said. Bail set for the men in unrelated cases was also revoked.

Barbosa’s bail was set at $10,000, and if he posts that sum, he must wear a GPS monitoring bracelet, prosecutors said.

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