A man was arrested in connection with the disappearance of a student who had been last seen on Boxing Day.
Joy Morgan, missing in police custody on February 7, was last seen in Ilford, east London.
A 40-year-old Londoner has been arrested and released on bail pending new information.
The agents described the case as a "complex investigation of a missing person".
Hatfield, a 21-year-old student at Hertfordshire University, is described as being black, of fine build, 5 feet 5 inches tall, with Afro-style hair that she often covers with a headscarf.
She also wears glasses.
His family made an appeal saying, "You love us, we miss you, wherever you are, send an SMS or call so that we know that everything is fine.
"No matter what happened, just come home."
It was Mrs. Morgan's 21st birthday on Tuesday.
It has links across London, including Battersea and Cricklewood, and in Bedfordshire.
Welwyn Hatfield's chief inspector, Tannis Perks, said: "This is a complex investigation into missing persons and we are working hard to establish the circumstances of the case. the disappearance of Joy.
"Of course, her family is extremely worried about her and desperate to know that she's fine." I'd like to appeal to Joy for her to contact us to let us know that she is safe. "
:: Anyone informed should call the police at 101. Immediate reports should be reported to 999.