Someone saw the drowning death of a 21-month-old child as a chance to cash in.

Jessica Struthers and Matt Bock of Blaine, Wash., had been mourning the death of their daughter, Ava, for less than a year when they found out someone stole her identity to claim her as a tax deduction.

Now ABC News reports the grieving couple must prove to the IRS that Ava really was their daughter.

The IRS wants information, but it's a one-way street.

"We were shocked. Who does this?" Struthers tells ABC News. "And of course, we want to know who, and they won't tell you."The culprit allegedly received a federal tax return worth several thousand dollars thanks to stealing Ava's Social Security number.

"It makes me sick," Struthers tells the network -- especially now that the burden of proof falls on her and husband, rather than the thief.

The couple lives in Blaine (in the northwest tip of Washington state on the Canadian border) with their three sons and a foster child they are working to adopt.

Ava was born in November 2007. ABC News reports she was was a twin who made a habit of running on the tips of her toes and stretching out the word "Daddy" with delight when he came home from work.

She died in August after she climbed into the family's backyard swimming pool and drowned.

"After she passed away, I feel like a big hole got cut into my heart," Bock tells ABC News.

Because of the theft of their daughters' identity, Struthers tells the network the family has to relive the tragedy day after day.

"You just don't want to talk about it every single day," she adds. "That was supposed to be the closing of the year and trying to move on. It just seems like we can't get past that."

ABC News reports the couple found out Ava's identity had been stolen when Bock's electronic tax filing was rejected about 48 hours after he submitted it. At the first, he tells the network, he thought there was some kind of glitch because of Ava's death.

Not knowing Ava had died, an IRS agent told Bock the easiest thing to do to resolve the problem would be to simply leave her off his own filing and try again next year.

"There is no next year," Bock remembers angrily telling the agent.

Children account for about 5 percent of all identity theft victims, Jay Foley, executive director of the Identity Theft Resource Center in San Diego tells ABC News.

The real number is hard to know, he adds, because parents may not notice for years -- if at all.

Death certificates, in most states, are public record. Anyone can search a death registry and find a dead person's name, date of birth, address and Social Security number.

Struthers tells ABC it angers her that the IRS put the burden of proof on her family without trying to nab the guilty party.

"Whoever claimed her does not have the right to claim her," she tells the network.

"I'd like them to go to jail. It's fraud," she adds. "They're messing with the IRS, but the IRS is just going to lay there and take it. They don't care."

Related: Identity Crisis - More Women Targets of Theft