The investigation of a car bombing that apparently targeted a high-profile Arkansas doctor turned to a tire left in his driveway, his role as chairman of the state medical board and lawsuits he was involved in.

Dr. Trent P. Pierce, 54, remained in critical condition after an explosive device planted in his Lexus blew up as he left for work.

Pierce lost an eye and suffered severe burns and shrapnel injuries in Wednesday's attack in his driveway in West Memphis, Ark.

A family friend, Scott Ferguson, said Pierce's family told him he'd bent down to move a tire blocking his hybrid Lexus SUV minutes before the explosion.

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Austin Banks, a senior special agent with the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives, said investigators want to learn more about the tire.

"This is a lead we're going to follow up on," he said.

Banks said residue and remains from the bomb had been sent to the agency's laboratory in Atlanta. Banks described the bomb as homemade, not manufactured dynamite or a military-style explosive.

The agent said bombs have certain characteristics that make them traceable, much like ballistics tests on bullets.

"The signature of the device ... pretty much identifies ... the maker," Banks said. "That's our job — to put it back together and find out who did it."

Agents were investigating lawsuits involving the family physician, an asthma specialist.

Until a few weeks ago, Pierce was a co-defendant with a nursing home operator dogged by dozens of allegations of abuse and neglect in a wrongful death case that last month made its way to the Arkansas State Supreme Court.

The lawsuit was brought by the estate of a deceased woman, Norma Louise Ramsey, against Pierce, the nursing home company formerly known as Beverly Enterprises — now called Golden Living — and other defendants.

Pierce served as medical director of the West Memphis nursing home run by the former Beverly Enterprises until 2004, when the facility was sold, company spokesman Blair Jackson told FOXNews.com.

Ramsey, 92, died in July 2004, according to her obituary in The Arkansas Democrat-Gazette. There was no cause of death given.

On Jan. 22, Pierce was dropped as a co-defendant after a judge granted his motion to dismiss an appeal in the case.

Click here to read the judge's decision to dismiss the appeal. (pdf)

In 2005, the state of Arkansas paid $1.5 million to settle 26 lawsuits brought against Beverly Enterprises. The suits stemmed from allegations of mistreatment and neglect of residents at 12 of its nursing homes.

Jackson told FOXNews.com on Thursday that he couldn't comment on the Ramsey case because it was still ongoing for the nursing home chain, but said Pierce doesn't have current ties to Golden Living.

"He has no affiliation with our company," said Jackson.

The company issued a statement about the attack on Thursday because of Pierce's former role as one of its medical directors.

"We were shocked by the bombing that injured Dr. Pierce, and we certainly wish him a speedy recovery from his injuries," Golden Living said. "Because the lawsuit that involved Dr. Pierce and names Beverly Enterprises along with other defendants is still pending, it would not be appropriate for us to comment on it at this time."

The chain has paid millions in wrongful-death lawsuits brought in several states in addition to Arkansas, including Mississippi, Florida, Virginia and Indiana.

In 2006, the Fort Smith, Ark.-based Beverly Enterprises was sued for $155 million in the neglect and eventual death of 84-year-old Loren Richards by his daughter and estate. The case went to trial.

In one such case in Evansville, Ind., involving the death of an 86-year-old woman who lived at one of the nursing homes, charges were brought against several individual caregivers and employees of the Beverly Enterprises facility. It went to trial, but all the charges were dismissed.

Beverly Enterprises/Golden Living owns and operates 325 nursing homes in 22 states.

Calls to the ATF, police and board attorney Trice about the lawsuit involving Pierce weren't returned.

Members of the Arkansas Medical Board attempted to carry on business as normal at their previously scheduled meeting Thursday, which Pierce had planned to attend. The meetings run through Friday.

ATF agents repeatedly pulled out board members for interviews as Little Rock police officers guarded the building.

Authorities pored over the panel's disciplinary records, paying close attention to its split decisions.

As a habit, Pierce only cast a deciding vote as chairman when the board's 12 other members couldn't resolve cases.

Local police said Thursday they didn't know who might want to harm the doctor.

"It's still a mystery to us. We have no idea," West Memphis Police Chief Bob Paudert told FOX News. "We have no leads at this point. As far as suspects, we have no direction to go."

Board member Joseph Beck questioned whether a board decision played any part in the bombing.

"Anytime a person's professional license is at stake, emotions run high, but that's been going on for years with this board," the Little Rock doctor said. "This was something atypical and terrible."

Thursday afternoon, Pierce could move his fingers and toes on command, as well as bend his knee, said Ferguson, the family friend and a West Memphis doctor.

Ferguson said doctors examined Pierce's right eye and believe he will be able to see out of it again.

Doctors placed an oxygen tube in Pierce's throat, which stops him from speaking with detectives, Ferguson said.

He is expected to undergo surgery again Friday to repair a broken wrist and insert a plate there. Previous operations cleared away shrapnel blasted into his neck and abdomen.

There's been a "remarkable change since they've done the cleaning and the debris removing in the burn areas," Ferguson said. "Yesterday, everything just looked charred and gone. Today, when they cleared everything away, you could see clean, healthy skin underneath."

The explosion, just before 8 a.m. Wednesday, could be heard a mile away in West Memphis, a small town just across the Mississippi River from Memphis, Tenn. Police impounded Pierce's Lexus as evidence, its bumper and grill partially torn away by the blast.

Pierce, who is married and has two grown children, was appointed to the state medical board in January 1997 and reappointed in 2005 by then-Gov. Mike Huckabee.

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FOXNews.com's Catherine Donaldson-Evans and The Associated Press contributed to this report.