A desperate search from air, land, and water for a 7-year-old boy who went missing at a city-run day camp in South Boston came to a horrifying conclusion Tuesday night when his body was found in the water off Carson Beach, police said.

Boston Police Commissioner William B. Evans told reporters at the scene that the Boston police Harbor Unit, along with Quincy police, found the body of Kyzr Willis of Dorchester shortly before 7:10 p.m. The child was located in the water behind the Curley Community Center, which includes the L Street bathhouse, police said.

Kyzr was attending the center’s camp, which runs from 10 a.m. to 3 p.m. Monday through Thursday.

“My heart goes out to the family,’’ Evans said. “This is a tragedy.’’

He said the bathhouse will remain closed until further notice while police investigate.

Evans added that authorities “put their heart and soul’’ into finding Kyzr.

“Obviously our hearts break for the family,’’ he said. “It’s everyone’s worst nightmare.’’

Evans provided no additional details about how the boy slipped away unnoticed by camp staff members.

Shortly before 7:40 p.m., family members sprinted into the community center and then began wailing inside.

Cries of “no!’’ and “terrible!’’ and, in Spanish, “My God!’’ could be heard above the sobs. One man shouted “My nephew’s gone!’’

Kyzr’s mother, Melissa Willis, and several other family members left the center at 8:30 p.m. after the devastating news with a police escort. They did not speak to reporters.

Sherie Seymore, a family friend and assistant pastor of the church the family attends, said they want to know how Kyzr evaded the watch of his camp counselors.

“Everyone wants to know,’’ she said. “If they saw him go into the bathhouse, how come they never saw him come out?’’

Kyzr was last seen at about 2:05 p.m. at the bathhouse, according to State Police.

State troopers searched the beach area for the child, and Boston police officers were going to check the boy’s home, State Police spokesman Dave Procopio said.

The search included State Police and US Coast Guard helicopters circling above the beach and troopers in boats combing the water. Beach lifeguards and a Boston Fire Department vessel also searched in the water.

Willis, 32, said earlier outside the bathhouse before Kyzr had been found that she dropped her son off for camp at 9:45 a.m. The camp is run by the Boston Centers for Youth & Families, a city agency.

Willis said her son did not show up when she returned to the bathhouse shortly before 3 p.m. to retrieve him.

“He’s a good kid,’’ she said. “I don’t know why he would ever go off. . . . He’s always with an adult.’’

She described her son, who was scheduled to attend first grade at the Mather Elementary School in the fall, as a boy with a kind disposition and a flair for excitement.

“He’s a daredevil,’’ she said. “He’s outgoing, he keeps you on your toes, and he’s just a great, intelligent kid.’’

Willis, who was joined by her husband, Ralph, fought to maintain composure as she reflected on the ordeal, while detectives conversed nearby.

Will Morales, commissioner of the Boston Centers for Youth & Families, said in a statement Tuesday night, “The entire BCYF community is absolutely heartbroken on the loss of Kyzr Willis and our deepest sympathies go out to this young boy’s family. BCYF will work closely with the Boston Police Department throughout the investigation.’’

The office of Mayor Martin J. Walsh said he was returning home early from the Democratic National Convention on Tuesday night, and the mayor expressed his condolences in a statement.

“The City of Boston is devastated to learn of the tragic loss of Kyzr Willis,’’ Walsh said.

“My heart is broken for the Willis family and my thoughts and prayers will be with them,’’ he added.

Evans told reporters earlier that authorities believed Kyzr walked out of the bathhouse by himself at about 2:15 p.m.

“He wandered out the front door from what we understand,’’ Evans said. He did not disclose how police received that information.

As the search wore into the evening, one woman, Lori Frankian, 49, sat in her wheelchair on nearby William J. Day Boulevard and held up a printed photograph of Kyzr for passing cars to view.

Frankian, who had just met members of the boy’s extended family on the street, expressed hope for his safe return.

“This child needs us, and we all have to come together and find him,’’ she said.

Late Tuesday night, the Willis family and about 30 friends and neighbors gathered on Trull Street in Dorchester to mourn the loss. The family sat on lawn chairs and wept, and people came up to them to express condolences.

Globe correspondents Olivia Quintana, Lauren Fox, Samantha J. Gross, and Bill Brett contributed to this report. Travis Andersen can be reached at travis.andersen@globe.com. Follow him on Twitter @TAGlobe.