A third arrest has been made in the shooting death of a Texas teen and her boyfriend, San Antonio police said. 

Myrta Romanos, 47, was charged with three felonies, including altering, destroying, or concealing a human corpse, abuse of a corpse and tampering with evidence, police announced Wednesday.

Her arrest comes one week after police charged Christopher Preciado, 19, with capital murder and Ramon Preciado, 53, with abuse of a corpse. Ramon Preciado's charge stems from allegedly helping Christopher Preciado move the bodies after they were killed.

San Antonio Police Department Lt. Michelle Ramos said at a news conference Wednesday that Romanos is "maybe a stepmother" to Christopher Preciado, and all three suspects lived together.

Savanah Nicole Soto, 18, and her boyfriend, Matthew Guerra, 22, were reported missing in late December before they were found dead in Guerra's car several days later. The couple disappeared a day before Soto, who was 9 months pregnant, was scheduled to be induced to give birth. Both Soto and Guerra were found with gunshot wounds.

Police said surveillance footage was a key factor in all three arrests. Romanos was seen leaving and returning to her home in a truck with Christopher and Ramon Preciado on the night of the murder, according to police. 

The gun used in the killings belonged to Romanos, police said.

Detectives were aware early in the investigation that a third person was involved but there was not enough evidence at the time to confirm it was Romanos, according to Ramos. 

Shortly after the murders, police released a video showing what they described as two persons of interest. One person was seen driving a dark Chevy Silverado with a bed cover, and another was seen driving Guerra's Kia Optima. San Antonio Police Department Chief Bill McManus said the video was recorded "very close to" where the bodies were found and a few days before they were discovered.  

"A lot of information on social media that there was a third individual involved, and our homicide detectives were aware of that, however, we were looking for enough probable cause to make that arrest," Ramos said Wednesday 

Ramos said San Antonio police are confident there are no more suspects in the killings.