A week ago, before her killer was arrested and made a full confession to her murder, the FBI issued a chilling warning in the Mollie Tibbetts case: the student's attacker is hiding in plain sight and we do not know who he is.

It was a frightening claim which was proven true on Tuesday when Cristhian Rivera, an immigrant farm worker who has lived near Brooklyn, Iowa, for up to seven years, admitted killing Mollie and led police to her body.

Now, as Rivera faces life behind bars or deportation and as Mollie's struggling family come to terms with her death, questions over where she may be have turned to how her murderer went undetected for weeks as her disappearance captivated the country.

Rivera was not known to police until Friday, when investigators traced him through his Chevy Malibu which was filmed following Mollie as she went on her evening jog on July 18. The footage came from security cameras at a private citizen's home.

Cristhian Rivera, 24, is shown left with Iris Monarrez, the 25-year-old who he shares a daughter with. Monarrez and Mollie Tibbetts (right) were Facebook friends

Police obtained them around a week ago while conducting one of thousands of canvass interviews. They spent days analyzing the footage before finally linking the vehicle to its owner.

He had no immediate link to Mollie but his ex-girlfriend Iris Monarrez was friends with her on Facebook and he told police after he was arrested that he had seen Mollie jogging before.

Monarrez has not commented on his arrest.

Rivera was arrested on Monday when police traced him through surveillance footage which showed his car following Mollie. He then made a full confession to killing her but his motive is not known

'He certainly wasn't known to the local police department.

'Through our interviews and conducting canvass interviews throughout the community we weren't aware of him until we were able to locate the vehicle.

'It's not like he was on anybody's radar. We weren't aware of him throughout the investigation,' Rick Rahn, special agent from Iowa's Division of Criminal Investigation told Today on Wednesday.

When he was approached by police on Friday, Rivera, a 24-year-old father, went willingly to participate in an interview.

He then confessed to attacking Mollie, putting her body in the trunk of his car and driving her to a field 10 miles away.

Afterwards, he led the officers to her body.

It was a stunning end to a five week long investigation which focused on others living in the town, including a pig farmer who was questioned three times and had his home searched.

Wayne Cheney, the farmer, allowed police to search his home and land and was subjected to a polygraph test.

This is the house where Rivera was living with other migrant workers before he was arrested

MOLLIE TIBBETTS FAMILY STATEMENT ON HER MURDER Our hearts are broken. On behalf of Mollie's entire family, we thank all of those from around the world who have sent their thoughts and prayers for our girl. We know that many of you will join us as we continue to carry Mollie in our hearts forever. At this time, our family asks that we be allowed the time to process our devastating loss and share our grief in private. Again, thank you for the outpouring of love and support that has been shared in Mollie's name. Advertisement

No other suspects or people of interest were ever named.

Mollie's boyfriend was out of town the night she went missing as was his brother, his brother's fiancee and her father Rob. Rob said he believed his daughter had been taken by someone she knew but that he believed she was still alive.

Special agent Rick Rahn said on Wednesday that the only reason Rivera was arrested was because of the footage

On July 18, he was seen driving around in his black car, following the route she took.

He told police that when he eventually approached her, he parked his car to get out and speak to her then attacked when she threatened to call the police.

It remains unclear when exactly Rivera entered the US. He is originally from the town of Guayabillo in the state of Guerrero.

There is no trace of him on property or voter records in the US but he had been working for Yarrabee Farms which is owned by Dane Lang, the brother of a prominent Republican Iowan.

He passed the background check required to get a job there and had been living with other migrant workers in a farmhouse that is owned by the company they work for.

He'd been living there for four years and was an employee 'of good standing'.

Little is known about Rivera's background aside from the fact he moved from Guerrero to the US as many as seven years ago

Police now say they are grateful for the person whose cameras caught his route on July 18 because without it, they would still be looking for the person responsible.

'We ere fortunate enough to be able to have that video.

'We spent a number of days and hours reviewing the video and subsequently were able to identify Rivera's vehicle and we were able to locate him and ultimately conduct the interview with Mr. Rivera,' Rahn said.

Mollie's parents are yet to speak about the discovery of her body or about Rivera's arrest.

He is due in court later on Wednesday afternoon and is also the subject of an ICE detainer.