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Editor’s note: For the latest on the McStay murders, watch “Buried Secrets: Who Murdered the McStay Family?” at 9 p.m. ET Tuesday on CNN.

It’s been more than four years since the McStay family went missing, and nearly eight months have passed since their remains were found in the Mojave Desert.

Who would want to kill Joseph; his wife, Summer; and their two young kids, Gianni, 4, and Joseph Jr., 3? How did their bodies end up in two shallow graves some 100 miles north of their home?

Investigators still have not named any suspects or persons of interest, but those involved aren’t giving up.

A new detective recently joined the investigation, bringing new energy and new ideas to the table.

As the investigation continues, here are five questions that could provide clues about the case:

1. Who used Joseph McStay’s phone shortly after he disappeared?

On February 4, 2010, Chase Merritt received a phone call from Joseph McStay. It was 8:28 p.m.

Chase, Joseph’s close friend, picked up the phone, looked at it, and set it back down.

“I had a bunch of other things I was doing, and I was just tired,” he told CNN.

Joseph ran a custom water feature business, and he often bought custom indoor waterfalls from Chase. The two talked frequently. Chase had already talked to Joseph multiple times that day, and they had also met in person for a couple of hours.

“I had no idea that something like this was going to happen,” said Chase.

Was that just a regular call from his friend, or could it have been a call for help?

“There are hundreds of scenarios. I have gone over all of them in my head,” he added. “Of course I regret not picking up the phone.”

2. Who used Summer McStay’s credit card the day she disappeared?

According to phone records, Summer McStay made a call from her home phone at 2:11 p.m. February 4 regarding purchasing herbal medicine. Financial records show her credit card was used 25 minutes later at a store in Vista, California, about 20 minutes from her home.

It’s not clear whether Summer was the one who made that purchase.

“That is certainly a piece of evidence that we would review if there’s any video or any documentation to support who was at that store and used that credit card,” said John McMahon, sheriff of San Bernardino County.

“Every piece of evidence in this case is critical,” he said.

3. Did Joseph McStay’s business have anything to do with his families’ disappearance?

In the months before his disappearance, business at Joseph’s company was good.

So good, in fact, that Joseph was working on a deal that could have been worth $9 million, his father, Patrick McStay, said.

By summer of 2011, Dan Kavanaugh, who worked for Joseph managing his company’s website, had sold the business to an outside company. Patrick was enraged.

“He owned nothing of — any part of, any share of, anything,” he said.

Dan, however, said he and Joseph split everything 50-50.

“We shared ownership from the beginning of starting the company,” he said.

Although Patrick had his suspicions about Kavanaugh, Dan has maintained his alibi and innocence.

“I was in Hawaii for over a month before he disappeared,” he said.

And the evidence CNN has uncovered seems to indicate Kavanaugh was in Hawaii around the initial days of the McStays’ disappearance.

4. Why was someone using the McStays’ computer to research traveling to Mexico?

One week before the McStays went missing, someone used their home computer to search for information on how to get children into Mexico.

Four days after they disappeared, detectives say the family’s white Isuzu Trooper was parked and subsequently towed from a parking lot just steps from the Mexican border.

Investigators also found surveillance video showing a family of four matching the McStays’ description crossing on foot into Mexico on February 8.

“I just thought, ‘Well, maybe they took off,’ ” said Joseph’s mother, Susan Blake. But his father wasn’t buying it.

“I said right up front, the first time I saw it (the surveillance footage), it wasn’t them,” said Patrick, adding that Summer was afraid of Mexico.

“Would Summer take her two children in there? Heck, no,” he said.

Still, investigators want to know whether the computer search and their disappearance were related.

5. Was crucial evidence in the McStay family home destroyed?

Eleven days after the McStays went missing, Joseph’s brother Michael called the Sheriff’s Department. He said he waited so long because he didn’t want to overreact and thought the family might just be on vacation.

The Sheriff’s Department immediately alerted homicide, but it took investigators four days to obtain the warrants needed to complete a full search of the home.

During those three days, the McStay home was unsealed. The McStays’ friends and family had some access in and out of the house.

Joseph’s mother straightened up the kitchen, which she says smelled terrible because of the trash.

Michael McStay said the house was not deemed a crime scene because there was no sign of forced entry.

With investigators’ permission, he said, he grabbed his brother’s computer and SD card.

Given all of the foot traffic in the house, freelance investigative journalist Steph Watts said critical evidence could have been lost.

“Certain items that might have been really key to the big mystery of why they left that house are gone,” he said.

CNN’s Melissa Dunst, Dana Ford, Jessica Small, and Kristi Ramsay contributed to this report.