The New York Police Department has shared a detailed timeline of the events surrounding the disappearances of two boys who were found dead in separate Manhattan rivers last week, while pleading with the public to come forward with any information that could help investigators determine what led to their deaths.

The agency is working with the city's chief medical examiner and the Manhattan District Attorney's Office in the investigation to determine "whether there was any criminality involved or if this was just a tragic incident," Chief of Detectives James W. Essig said during a Wednesday news conference.

"If anyone has seen any video or heard of any video, please contact us. You are not in trouble for not coming forward in a timely manner," Essig said.

Alfa Barrie, 11, and Garrett Warren, 13 -- who attended separate schools but were believed to be friends -- were seen together in Harlem on May 12 before they were reported missing at different times, according to the NYPD.

The department's harbor unit found the bodies of the boys days apart, more than two miles away from one another in the Harlem River and the Hudson River.

Garrett died by accidental drowning, and the cause and manner of Alfa's death is pending following an examination Sunday, according to the medical examiner's office.

The timeline shared Wednesday was based on video recovered during the investigation, eyewitness accounts and interviews, police said.

May 12: Boys seen together near park, shoreline

Shortly after noon on May 12, Alfa was dismissed from Democracy Preparatory Charter School in Harlem, Essig said.

The boys met up around 3:30 p.m. and police have placed them at several locations together in Manhattan over the next couple of hours, including "a fish market, exiting a residence and at a deli," according to Essig.

Shortly before 7 p.m., the boys were seen riding bikes and meeting up with a third youth, police said. Video footage taken from a distance shows two of the boys climbing over a fence near the Harlem River around 6:50 p.m. and heading towards the shoreline, Essig said.

The next time police observe anyone on video is about 14 minutes later, when the third boy is observed entering a park, called the Brigadier General Charles Young Playground, Essig said. Roughly two hours later, around 9 p.m., that third youth is seen leaving the park.

Five minutes after that boy is observed, police received a 911 call from an anonymous man who said that "a group of kids approached him" and then stated, "two kids were pushing each other and fell in the water," Essig said.

The 911 caller engaged with the teenagers while he was on the line with police, attempting to get more information, Essig said. When police arrived at the scene, the 911 caller -- who police were not able to reach -- and the three children had already left, according to Essig.

The police called in harbor and aviation units to conduct a thorough search but were not able to spot either boy in the water, Essig said. Police also canvassed the surrounding area and the park, which was very crowded, but found no witnesses, he added.

A law enforcement source previously told CNN authorities have an account from another boy who was reportedly at the scene, who said Alfa and Garrett were playing by the water's edge before one boy shoved the other toward the water and they both ended up going in.

The law enforcement source said the third boy saw them briefly but did not see them come up again or climb out of the water. A couple of hours later, he approached an adult, who called 911 and described the scene to police, the source said.

May 14 and 15: Family members report boys as missing, search begins

On Sunday, May 14, at 2:23 a.m., Alfa's sister called 911 and reported him missing.

On May 15, at 1:25 p.m., Garrett's mother reported him missing, which led detectives to "quickly determine" the two missing boys were connected, Essig said.

"A command post was established with multiple units canvassing buildings, parks and businesses," Essig said. "Aviation, harbor and scuba were on the scene and conducted river and coastline searches."

Later that evening, while the search was ongoing, detectives identified and questioned the third youth, Essig said.

Over the next few days, several young people in the park were interviewed by detectives, but none of them were eyewitnesses to the incident, Essig added.

May 18 and 20: Bodies found, days apart

Garrett was found May 18 at 10:25 a.m., in the Harlem River near the Madison Avenue Bridge, which connects Manhattan and the Bronx.

On May 20, at 6:50 a.m., police found Alfa's body in the Hudson River near the Upper West Side, according to police.

The bodies may have been found in the two rivers due to differences in decay, water temperature and depth, as well as body composition, according to Inspector Anthony Russo, commanding officer of the harbor. The bodies likely surfaced at different times, when the water currents were traveling south and north, respectively, Russo said at the news conference.

"We want to find out exactly what occurred on that river. We owe that to the family for closure and we also owe it to the public, as well," Essig said on Wednesday.

CNN affiliate ABC 7 reported family and friends gathered at a mosque in the Bronx after Alfa's body was found.

Family friend Adhmadou Diallo said the boy was close with his mother and helped out at home. "Every time he gives his mommy a goodbye kiss when going to school -- and that's what happened on Friday when he left," Diallo said.

Alfa was a sixth-grade student at Democracy Prep Public School in Harlem, which released a statement mourning the loss of the "funny, bright, witty, inquisitive, and brave child," ABC 7 reported.

"He will be sorely missed by his teachers and fellow students," the school said. "We also stand in solidarity with the family of Alfa's friend, Garrett Warren. While he was not a student, we understand that this is a difficult time for his community as well and share our prayers with them."