Invasion of the Body Snatchers

In deep space, a race of gelatinous creatures abandon their dying world. They make their way to Earth and land in San Francisco. They fall on plant leaves, assimilating them and forming small pods with pink flowers. Elizabeth Driscoll (Brooke Adams), an employee at the San Francisco Health Department, is one of several people who bring the flowers home.
The next morning, Elizabeth's boyfriend, Geoffrey Howell (Art Hindle), suddenly becomes distant, and she senses that something is wrong. Her colleague, health inspector Matthew Bennell (Donald Sutherland), suggests that she see his friend, psychiatrist Dr. David Kibner (Leonard Nimoy). While driving to Kibner's book party, they are accosted by a hysterical man (Kevin McCarthy). The man runs off, and is soon seen dead, surrounded by a crowd of emotionless onlookers. At the party, Matthew calls the police about the incident, and finds them strangely indifferent. An agitated party attendee starts declaring that her husband is not her real husband. Kibner works to reconcile them, and suggests that Elizabeth wants to believe that Geoffrey has changed because she is looking for an excuse to get out of their relationship.
Meanwhile, Matthew's friend Jack Bellicec (Jeff Goldblum), a struggling writer who owns a bathhouse with his wife Nancy (Veronica Cartwright) discovers a deformed body on one of the beds and calls Matthew to investigate. Noticing that the body (which is adult sized but lacks distinguishing characteristics) bears a slight resemblance to Jack, Matthew breaks into Elizabeth's home and finds a semi-formed double of her in the bedroom garden. He gets the sleeping Elizabeth to safety, but the duplicate body has disappeared by the time he returns with the police. The body at the bathhouse has also disappeared when Matthew returns there.
Matthew realizes that people are being replaced by extraterrestrial copies while they sleep. Matthew calls several state and federal agencies, but they all tell him not to worry. In addition, people who had earlier claimed that their loved ones had changed seem to have been converted as well, including (unbeknownst to him) Dr. Kibner.
That night, Matthew and his friends are nearly duplicated by the pods while they sleep. The pod people try to raid Matthew's house, but he and his friends are able to escape. During this, they discover that the pod people emit a shrill scream once they learn someone is still human among them.
Jack and Nancy create a diversion within a crowd of pursuing pod people to give Matthew and Elizabeth time to escape. Matthew and Elizabeth are chased across San Francisco. They are eventually found by the doubles of Jack and Dr. Kibner at the Health Department building. Kibner's double tells them that what the alien species is doing is purely for survival and that they are even doing humanity a favor by ridding them of emotion. Matthew and Elizabeth are injected with a sedative to make them sleep. However, having already taken a large dose of speed, the couple overpower them and escape the building.
In the stairwell, they find Nancy, who has learned to evade the pod people by hiding all emotion. Outside, Matthew and Elizabeth are exposed as human when Elizabeth screams after seeing a mutant dog with a man's face. They flee, and discover a giant warehouse at the docks where the pods are grown. After Matthew and Elizabeth profess their love for each other, Matthew goes out to investigate, only to discover a cargo ship being loaded with hundreds of pods.
Matthew returns to find that Elizabeth has fallen asleep. He tries to wake her, but her body crumbles to dust and her naked double arises, telling him to embrace his fate and sleep. Matthew returns to the warehouse and sets it on fire, destroying many pods. He hides from the pod people under a pier, but they know he will have to fall asleep eventually.
The next morning, Matthew watches dozens of children being led into a theater to be replaced. At work he sees Elizabeth, but she is completely oblivious to him. While walking towards City Hall, he is spotted by Nancy, who has remained human. She calls to him, but he responds by pointing to her and emitting the piercing pod scream. Realizing that Matthew is now a pod person, Nancy, now the only human left in the city, screams in helpless terror.

Question: Who is chasing Matthew and Elizabeth?
Answer: Pod people