But where’s the girl?

She is about 13. A cigarette dangles between her fingers and a smattering of tattoos adorn her skin -- three dots on her lower back and, on her ankle, something that looks as if it could be a butterfly, maybe a hummingbird. A blue heart-shaped sticker is pasted next to the outside corner of each eye.

Todd Hammer has been searching for the girl for two years now, with only a few dozen pixelated photos a decade old to go on.

The investigator knows every corner of the apartment she’s in -- the shuttered vertical blinds, white walls, dark Berber carpet, old-fashioned wall-mounted gas furnace. He has memorized the mundane details: a copy of the Yellow Pages, a language school flier, an old wall calendar from a Jewish religious supply store in Encino.

He studies the two adults in the photos: a man with a pot belly, widow’s peak and graying around the chin, his face obscured by a black oval. A woman with an eyebrow piercing and a tattoo of a sleeping cat behind her shoulder.

The girl must be an adult now, but the crimes haven’t stopped. Month after month, police across the country and on different sides of the planet discover that the photos of sex acts are in the hands of yet another child pornography collector -- hundreds of them by now.

Finally, a long-awaited break. On Friday, authorities announced the arrest of the woman believed shown in the photos. She stands accused of distribution and production of child pornography.

Will the woman finally lead him to the girl?

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The ‘Jen Series’

The girl is a suspected victim of sexual abuse depicted in widely circulated child pornography images known as the “Jen Series,” a set of forty-some photos first discovered by investigators in the Chicago area in 2007.

Hammer, a child exploitation investigator with Immigration and Customs Enforcement in Los Angeles, is the latest on the trail. Finding the girl would have a hand in prosecuting collectors of child pornography across the country, cases that could number in the hundreds.

Identifying the abused in child pornography cases took on a new importance for law enforcement in 2002. That’s when the Supreme Court ruled that unless the target of sexual acts is proved to be a child -- not a youthful-looking adult, not an adult digitally morphed to look underage -- the material is not illegal and is protected under the Constitution as 1st Amendment speech.

It’s a needle-in-the-haystack search for children who could be anywhere on the planet, a search in which anything from electric sockets unique to certain parts of the world or local programming flickering on a television screen can offer clues, and cadres of experts as unlikely as dermatologists, pediatricians and optometrists end up putting investigators on the right track.

In one New Jersey case, a botanist told investigators that plants in the background of a child pornography series were found only in a particular region of Thailand. The man in the photos was arrested in 2008 and admitted to production of child pornography and traveling to Thailand to have sex with boys.

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No traces

At a small Jewish store in an Encino strip mall, a husband and wife who run the religious supply store tell Hammer they handed out only a few hundred of the calendars in the photos, and only to walk-in customers. That tells him he’s searching the right area.

At the language school’s Northridge campus, none of the officials recognize photos of the girl or the woman. The girl, who a forensic pediatrician said was probably between 11 and 14, was too young to have been a student.

From the sequence of dates visible in the calendar, Hammer theorizes the photos were probably taken in the spring of 2001. After digging through the Web, he manages to find on EBay the same copy of a Men’s Health magazine on a table in the background of the photos. He sees the issue is from September 2000. That lends support to his time estimate.

Hammer pays a visit to the Mary Magdalene Project, a Van Nuys group that helps girls and women off the streets. No one recognizes the girl. He goes from school to school in the area, flipping though yearbooks from the right time period, looking for matches to the face now seared onto his brain.

He looks through logs of unidentified bodies at the county coroner. A long shot, he knows.

There are no traces of the girl.

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Instant sharing

Once, trading child pornography meant that photos or videos were sealed in envelopes and physically mailed from one collector to the next. In today’s world, massive amounts of data circumnavigate the globe in fractions of a second.

If a particular image or series of images has been reported more than five times by law enforcement, the National Center for Missing and Exploited Children, a nonprofit authorized by Congress to act as a clearinghouse for such material, considers them “actively traded.”

Images in the “Jen Series,” investigators say, have been reported about 300 times.

If he identifies the girl, Hammer would be called to testify in cases across the country to attest that the girl exploited in the photos is a minor. She would be entitled to her rights as a victim in each case, to testify about the impact on her life and ask to be compensated for her pain and suffering.

It’s a type of exploitation, one victim known as “Vicky” said in court statements, that is “never-ending.”

“Every day, people are trading and sharing videos of me as a little girl being raped in the most sadistic ways,” she wrote as a 19-year-old. “They don’t know me, but they have seen every part of me. They are being entertained by my shame and pain.”

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A dead ringer

Hammer turns his focus onto the adult woman in the images. Finding her, he thinks, could be key to breaking the case open.

He begins combing arrest and booking records in Los Angeles County. After narrowing the search with what he knows about the woman -- race, approximate age, locations of her tattoos -- he finds a booking photo that sends his heart racing.

A woman recently released from custody, with a string of drug and prostitution arrests, stares back at him from his computer screen. She’s a dead ringer for the woman in the “Jen Series.”

With no current address, Hammer heads to a stretch of Long Beach Boulevard frequented by street walkers where the woman was known to hang out. Her photo in hand, he drives up and down the street each evening as darkness sets in, scanning faces. He asks patrol officers, shelter volunteers and people on the street if they recognize the woman.

Two weeks into his queries, the trek pays off. A nonprofit worker recognizes the woman. She points him to an associate of the woman, who in turn gives him an address.

He knocks on the door of a ramshackle house converted into apartment units, and the door swings open. It’s her, he thinks, but he needs confirmation. He would need to get a look at the back of her right shoulder, for the tattoo of the sleeping cat.

He comes up with a ruse on the fly. He is investigating a kidnapping, he tells her -- people are always eager to help in those searches. He is looking, he says, for a woman with a large tattoo of the Virgin Mary covering her back.

Without a moment’s hesitation, the heavy-set woman flings off her shirt, under which she is wearing nothing. She turns her bare back to him.

There is no cat tattoo. It is another dead end.

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Hurt? Alive?

In the photos, the girl is forever 13. But where is she now? What kind of life has she led? Is she hurt, scarred?

Alive?

This week, in a last-ditch effort, authorities filed “John Doe” and “Jane Doe” warrants charging the man and woman in the photos with child pornography production, and turned to the public for help. They released photos of the adult suspects, black oval and all, before a phalanx of cameras.

Within hours, phone lines rang. Four callers pointed to the same woman: 52-year-old Letha Mae Montemayor of North Hills, who liked to be called “Butterfly.” Hammer kept his expectations in check all the way to the woman’s doorstep.

Disheveled and in flip-flops, the woman indignantly told the agents who met her outside her apartment complex that they had the wrong person. Her tattoos led authorities to believe she was the woman in the photos.

She now sits in federal lockup in downtown Los Angeles, scheduled to appear in court Monday. With the break in the case, Hammer will begin pounding the pavement anew -- to find the man, and perhaps, finally, the girl.

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victoria.kim@latimes.com