A woman drowned her infant son and his 5-year-old half-brother in a bathtub hours after she called police and told them she was worried about being deported, a Delaware police chief said Tuesday.

Kula Pelima, a native of Liberia who has lived in the U.S. for two decades, called 911 about 3:45 a.m. Monday, fearing that her visa had lapsed, Wilmington Police Chief Robert Tracy said at a news conference.

"She was concerned about her immigration status," Tracy said. "She's a Liberian immigrant that came to the country in 1997, and she was worried about what could happen to her because her boyfriend ... [the father of both boys] was picked up also by the feds on some immigration issues on Oct. 6."

It’s not clear why Pelima was worried. Immigration and Customs Enforcement said she is a permanent resident with no criminal convictions.

An officer visited her home early Monday and told her local authorities were not going to arrest her, the police chief said. The officer even gave Pelima an immigration hotline number to call, and observed the older boy doing well during the visit.

According to court papers obtained by FOX 29, Pelima told police she dropped the 4-month-old in the tub turned on the water and left him on his back as the tub filled early Tuesday.

Police said she then physically assaulted the 5-year old, dragged him to the bathroom and put his head under water until he stopped moving.

"She was pretty matter of fact about admission that the two children were dead. Our officers discovered them in the back bathtub," Wilmington Police Department Chief Robert Tracy said, according to FOX 29.

Tracy said the crime was shocking even to hardened officers.

Pelima, 30, was charged with two counts of first-degree murder and was being held on $2 million bond. It’s not clear if she has a lawyer yet.

There was a strong odor of gas in her apartment, but it was not clear if the gas stove had been turned on intentionally, the chief said. The building was evacuated but no one was hurt by the fumes.

Pelima’s boyfriend, the father of both boys, is a native of Nigeria. He was taken into custody by immigration authorities in Pennsylvania on Sept. 14. The couple lived together in Wilmington, and she helped care for both children, the police chief said.

Victor Epelle, 38, was in the United States legally, but he had violated the terms of his status, ICE said. It wasn’t immediately clear what he had done.

He was released from custody Monday on humanitarian grounds after being notified of the deaths of his sons.

The Associated Press contributed to this report