Lake County Major Crime Task Force Commander George Filenko said the men were captured on video taken by a home security system and a truck driver who had a camera in his vehicle the day Gliniewicz was killed.

"We have confirmed at this point they were not involved in this," he said. "Those individuals have established their whereabouts in that time frame." He did not identify the men.

Filenko also said DNA evidence was recovered at the crime scene that did not belong to Gliniewicz.

Investigators are comparing the DNA to a national database, he said. About 50 interviewed suspects have been swabbed for DNA, he said.

Thousands of mourners, including hundreds of police officers, attended the officer's funeral in Antioch, Illinois, on Monday

Gliniewicz was killed on the morning of September 1 in Fox Lake, a community of about 10,000 people in northern Illinois.

Gliniewicz radioed to dispatchers that he was following three men -- two white and one black -- into a heavily wooded area. He was later found shot dead there.

The lieutenant made the first call at 7:52 a.m. and called three minutes later to request backup. The backup units arrived at 8:01 and found their fellow officer dead, roughly 50 yards from his vehicle, at 8:09, Filenko said.

Authorities searched across tricky terrain and brought in helicopters, dogs, federal agents, night-vision equipment and body-heat sensors. Police cleared every home in the cordoned-off area and fielded more than 100 tips, Filenko said.

So far, police have found no witnesses.

Few details released

Authorities have disclosed few details about the shooting. The rough description of the three men is about all they have told the public about Gliniewicz's death.

"We're not prepared to give out any specific descriptions yet," Filenko told reporters last week.

Filenko confirmed at a news conference last week that the officer's .40-caliber pistol was found at the scene.

A source involved in the investigation told CNN on Thursday that Gliniewicz's gun was fired, though it's not clear who pulled the trigger.

The commander told reporters that investigators were using machetes and magnets to search the high grass at the scene of the slaying.

An autopsy was completed, but authorities aren't releasing the results yet, Filenko said. The Northern Illinois Police Crime Laboratory is expediting its review of evidence to determine whether there was any fingerprint or DNA transfer.

Local police officers have volunteered to come in while they're off duty, and other law enforcement officers in the surrounding area have called to offer their assistance, Filenko said.

Who was Joe Gliniewicz?

A local hero

Gliniewicz was a hero to many in Fox Lake. Hundreds of people gathered on September 2 for a vigil to honor the 30-year police veteran. He led the local police explorers' program, mentoring and training youths interested in becoming officers.

He was a husband and father of four.

"Joe was my best friend and my world. My hero," Melodie Gliniewicz told more than 1,000 people at the candlelight vigil.

She said her husband was "the love of my life for the last 26½ years. He was my rock as much as I was his rock."

America's fallen officers

CNN's Holly Yan, Deborah Feyerick, Steve Almasy, Tina Burnside, Eliott C. McLaughlin, Ryan Young and Rosa Flores contributed to this report.