Omar Khadr took the witness stand in Guantanamo Bay Thursday afternoon to tell the widow of a soldier he admitted to killing that he was "really, really sorry for the pain I caused you and your family."

Khadr made the statement just hours after the widow, Tabitha Speer, gave a victim-impact statement to a military jury, in which she described the "generous, loving" nature of her late husband, U.S. Army Sgt. 1st Class Christopher Speer.

"I wish I could do something that would take this pain away from you," Khadr told her.

Speer shook her head at first as Khadr spoke, then wept.

Christopher Speer was hit by shrapnel after Khadr threw a grenade during a July 2002 firefight in an Afghan compound. He died from his injuries 12 days later.

Khadr was captured by U.S. forces after the battle. He has been in U.S. custody ever since.

During sentencing Khadr said his biggest dream was to get away from Guantanamo Bay.

He said during his time there he has learned about the "wonders and beauties of life" and wanted to go experience those things for himself.

Khadr also said he wanted to become a doctor because he knew what pain was and wanted to relieve the pain of others.

When asked if he had anger in his heart, Khadr said he did not, CTV's Paul Workman reported.

"You're not going to gain anything by it," Khadr said, wearing a dark suit and tie.

"Love, forgiveness are more constructive and bring people together."

On Thursday, Canadian Foreign Affairs Minister Lawrence Cannon said there was no agreement to bring Khadr back to Canada.

He added that Canada was not involved in any plea deal between Khadr and U.S. prosecutors.

Widow's statement

Tabitha Speer told the jurors she had travelled to a military hospital in Germany to be with her husband after he was injured in the firefight.

Before leaving for Germany, Speer promised the couple's niece and their daughter Taryn -- then three-and-a-half years old -- that she would bring the injured soldier home.

"I broke that promise," Speer told jurors Thursday, describing the aftermath of her husband's passing.

When she told young Taryn about what happened, "she let out a scream," Speer said.

"A part of my daughter died with my husband."

Referring to Khadr, Speer said "someone who is so unworthy stole all of this from her."

Speer told jurors that her husband had a close relationship with his young children prior to his death.

Christopher Speer had a love of Elvis he shared with his daughter, Taryn, and he also left behind his son Tanner, who was only 10 months old when his father died.

When her husband was alive, "he did as much for the caring of the children as I did," Tabitha Speer said.

On Thursday, Speer read letters from her children describing the anger they feel towards Khadr to this very day.

"I'm mad at you because of what you did to my family," Speer said, when reading out a letter written by Taryn, who is now 11 years old.

"You make me really sad. I'm mad at you because of that."

Speer's son, Tanner, also wrote a letter for the court requesting that Khadr "go to jail because of the open hole he made in my family by killing my dad."

Khadr could be rehabilitated: lawyer

Earlier Thursday, the jurors heard testimony from a senior lawyer who used to work at the Guantanamo Bay prison.

Navy Capt. Patrick McCarthy worked at Guantanamo Bay for more than two years, from May 2006 to July 2008, in the camps where the prisoners are kept. He then served as the top military legal adviser at the facility.

During his time at Guantanamo, McCarthy said he did not see any signs that would suggest Khadr had been radicalized. Instead, he remembers Khadr as a polite and respectful inmate.

"Mr. Khadr was always very respectful," McCarthy said Thursday, testifying by video link from Afghanistan where he is now stationed.

"He had a pleasant demeanor. He was very friendly."

Furthermore, Khadr did not attack the Guantanamo guards as other "radical" detainees did, said McCarthy. He also helped calm the tensions brewing between the inmates and officials at Guantanamo.

McCarthy said it may be possible for Khadr to be rehabilitated because of his young age.

He appeared critical of the fact that Khadr had been jailed since the age of 15.

"Fifteen-year-olds, in my opinion, should not be held to the same level of accountability as adults," said McCarthy.

He also noted that Khadr was a young teenager when he was brought to Afghanistan by his father, Ahmed Said Khadr, who held close ties to Osama bin Laden.

"The fact that his father took him to become involved with the al Qaeda leads me to believe that he has rehabilitative potential," McCarthy said.

McCarthy's impressions of Khadr contrast greatly with those of Dr. Michael Welner, a psychiatrist who testified for the prosecution at the sentencing hearing earlier this week.

Welner described Khadr as being an unrepentant terrorist who "marinated in radical jihadism" while in custody over the past eight years.

McCarthy testified for the defence three days after the 24-year-old Canadian pleaded guilty to five war-crimes charges.

McCarthy said he did not have knowledge of the offences Khadr admitted to earlier in the week.

When the prosecution and defence are finished calling witnesses, the jurors will be given a chance to hand down a sentence to Khadr.

However, a plea agreement that Khadr has signed will ensure that he serves no more than eight years in prison. If the military jury hands him a lesser sentence, however, Khadr will serve that instead.

Khadr's plea agreement will also allow him to seek transfer to a Canadian prison after he has served one year of his sentence in U.S. custody.

The full details of Khadr's plea agreement have not yet been made public.

With files from The Associated Press and The Canadian Press