MARIETTA, Ga. — The suburban Atlanta father whose toddler died in June after being left in a hot vehicle was indicted Thursday on murder charges.

The indictment against the man, Justin Ross Harris, by a Cobb County grand jury includes a charge of malice murder, for which a death sentence can be sought. Mr. Harris also faces counts that include felony murder, cruelty to children and dissemination of harmful material to minors.

Mr. Harris, who was arrested hours after his 22-month-old son, Cooper, died on June 18, has been held without bond.

The Cobb County district attorney, D. Victor Reynolds, said his office would decide this month whether to pursue the death penalty. "We are pleased with the pace and the thoroughness of this investigation, which continues on today," Mr. Reynolds said at a news conference. "The evidence in this case has led us to this point."

He declined to answer questions and said, "The indictment speaks for itself."

Later Thursday, Mr. Harris's lawyer criticized what he said was the shifting "maze of theories" that the authorities had put forward to explain the defendant's behavior.

"Ross doesn't have any theories, just the truth," the lawyer, Maddox Kilgore, said. "And the truth doesn't need a flow chart to follow."

He added, "The truth is, Cooper's death was a horrible, gut-wrenching accident."

The grand jury's decision came after an onslaught of sordid revelations and wide swings in public opinion about the actions of Mr. Harris, whose supporters say he made a heart-rending error when he left Cooper in his car seat while he worked a corporate job at Home Depot. In the days after his arrest, Mr. Harris received support from thousands of people who signed online petitions that accused the authorities of overreacting to a mistake.

But in court filings and testimony, the authorities outlined what they said was a pattern of conduct showing that Mr. Harris had abandoned Cooper in a quest for a "child-free life."

Mr. Harris, the authorities said, did not have a typical workday as a computer specialist on the Wednesday when Cooper died. Instead, officials said, he exchanged explicit messages with six women and did nothing to help his child when he returned to his 2011 Hyundai Tucson around midday.

Mr. Harris's previous communications with a girl are at the center of three of the charges against him, one of which alleges that he sent her "printed matter containing explicit and detailed verbal descriptions and narrative accounts of sexual excitement and sexual conduct."

Mr. Harris is also accused of sending the girl a nude photograph and asking her for a sexually explicit image.

But the focus of the indictment is the death of Cooper, who, the indictment said, suffered "cruel and excessive physical pain" before he died. He is, according to statistics maintained by a researcher at San Jose State University, one of 26 children who have died this year in a hot car.

In Maryland on Thursday, prosecutors charged John M. Junek with involuntary manslaughter after the death the day before of his infant son in a hot car at Naval Air Station Patuxent River. An investigator wrote in a court document that Mr. Junek acknowledged driving to his office without taking his son to the base's child care center on a day when the temperature eventually reached 85 degrees. The authorities said the child was in the vehicle for more than six hours before he was discovered.

In the Georgia case, Mr. Reynolds, the district attorney, left open the possibility that others, including Mr. Harris's wife, Leanna, could be prosecuted, but he did not elaborate. Ms. Harris's lawyer, Lawrence J. Zimmerman, on Thursday questioned Mr. Reynolds's timeline for action.

"I am surprised that the district attorney is still contemplating, after almost three months of reviewing the evidence, whether or not to charge my client, if that is who he was referring to in his press conference," Mr. Zimmerman said in an email.

Mr. Reynolds said that Mr. Harris, whose lawyer said he was "doing terrible," would probably be arraigned this month.