Dr. Henry Lee, noted forensic expert
Dr. Henry Lee is the founder of the Henry C. Lee Institute of Forensic Science at the University of New Haven. (2007 photo) (Paul Buck, AP)

NEWTOWN, Conn. -- Adam Lanza, the 20-year-old Newtown man suspected of killing 20 children and six adults last Friday inside Sandy Hook Elementary School, killed himself as police were arriving, leaving far more questions than would likely exist had he been questioned by police.

Investigators, though, are entering their fifth day of a probe that is likely to shape national policy.

The investigation doesn't have to deal with some of the most difficult aspects of putting together a case because "basically the suspect already died," said Henry Lee, the retired head of the Connecticut state crime lab and perhaps the best-known forensic scientist in the world.

Had suspect Adam Lanza not committed suicide, shooting himself as he heard police approach the school, "it was going to be much more work to do," Lee said, because authorities would be putting together a criminal case.

Lee is no stranger to tragedy in Newtown. His forensic investigation helped convict Richard Crafts of killing his wife, Helle Crafts in what is commonly known as the "Woodchipper Murder" case and trial. Nationally, he is remembered for his testimony in the O.J. Simpson murder trial.

Officials will put together a detailed psychological profile of Lanza, which will likely shape the conversation on issues like gun control and treatment of the mentally ill.

"I think, because of the magnitude of the case, we're looking at a series of events that will change national policy," said John DeCarlo, a former Branford police chief who's now an associate professor at the Henry C. Lee College of Criminal Justice and Forensic Science at the University of New Haven. "I think the FBI and ... the government are going to be looking for much more."

Many questions about Adam Lanza's mental state, past and how he acquired weapons haven't been publicly answered.

The nature of details released will likely set the tone for future conversations on how to prevent a similar mass shooting.

"I think that right now, the biggest piece of the puzzle is in the understanding, in the motive, and I think as they work the case and come to an understanding, we'll see what caused this," DeCarlo said.

Lee said investigators are going to look for "any writing, any notes, any so-called 'unwritten' notes such as the type of emotional mood change that often could be overlooked."

"That's what we call unwritten language," he said. "Sometimes they do not write a note, but they provide a sign."

What Lanza did "is not a spur-of-the-moment decision" in which he simply "snapped," Lee said.

"He had to do some planning," he said.

The fact that he killed his mother, Nancy Lanza - shooting her as she lay in bed - before heading to the school "is a message," Lee said. "Basically, you kill your parents, it either comes out of love or out of hatred," he said.

A lot of the questions investigators will seek to answer will likely involve family issues, Lee said.

"Why did he choose to live with his mother and his brother went to New Jersey?"

"Of course these mass killings, in my career I've done a lot of cases...the suspect always has some psychological problems," Lee said.

Among the things that they're most likely looking at is "the fact that he's on the (autism) spectrum" and details such as whether Lanza's mother's gun safe was locked, he said.

"Some may be useful and others won't," DeCarlo said. "They're going through a large vetting process."

Authorities have released few details about the investigation, but a multitude of local, state and federal authorities will be sifting through information.

"Cases like this, which are horrific, usually get a huge amount of police attention, including from people from other areas who are brought in to help with the investigation," DeCarlo said.

There are multiple parties involved in the examination of any crime scene. Forensic technicians and investigators look for evidence around the crime scene while members of a chief medical examiner's or coroner's office examine bodies for a cause of death and potential evidence, said Victor W Weedn, chairman of the Department of Forensic Sciences at The George Washington University.

Medical examiners will often look into a deceased person's medical records to determine a cause of death. But the process can be expedited.

"In a case like this there is not a lot of time and there is a lot of public pressure to get things done," he said.

The cause of death in a shooting such as the one at Sandy Hook Elementary School is more apparent and the history of the deceased doesn't have to be examined as fully as other cases, he said.

Some assistance might come from the Violent Criminal Apprehension Program based in Quantico, Va. ViCAP, maintaining the largest investigative repository of violent crime information in the country, is a behavioral science unit profiling serial killers and violent criminals.

Its experimental psychologists help uncover the "why" behind crimes, DeCarlo said.

State police spokesman Lt. J. Paul Vance has said the school could remain in police custody for months. And that's partly because of the amount of work going into evidence documentation, but also as a precaution: Detectives after leaving the scene must sometimes go back to find answers to new questions, said Peter Valentin, a veteran crime scene investigator and a forensic science professor at the University of New Haven.

Having retired as a state police Major Crime Squad detective around a year ago, he offered some insight into work that goes on after a tragedy.

"Things that by all accounts are probably meaningless, but you just don't know," he said. "Like a cabinet door is open, are lights on in the room, how many chairs are there, how are they positioned."

It's likely that the same team is working day after day at the school because too many different people coming in and out can contaminate a scene. Also, there isn't a need to brief people new to the scene, he said.

Valentin recalled working 20-hour days and sleeping in his car while investigating the 2009 killing of Yale doctoral student Annie Le.

At the crime scene, detectives will determine Lanza's movements and proximity to victims, along with where he may have been standing when shooting. The locations of cartridge cases or the way bullets are embedded in a wall can help approximate where a shooter was situated, Valentin said.

Blood droplets or splatters will likely also be studied. The shape and size of droplets will help investigators figure out how the bullet hit to create that blood pattern.

"There are calculations you work on based on the droplet," he said. "You calculate angles and take string and use it to approximate the angle and show graphically what position the blood originated from."

Once evidence is seized from the site, investigators will have numerous tools at their disposal for further study.

Reporters Susan Misur and Katy Petiford contributed to this report.