One month after ordering the Trump administration to return a woman who had been deported to the Democratic Republic of Congo, a federal judge reversed his decision, citing new evidence.

The judge, Richard J. Leon, ruled in May that the Trump administration had improperly deported the woman despite a letter from the Congolese government refusing to take her on medical grounds.

But after weeks of court debate, the judge ruled on Friday that the letter had been sent to the U.S. government through “an irregular chain of custody” — a member of Congress and the woman’s lawyer — and that senior immigration officials and diplomats had been unaware of it before her deportation.

The woman, Adriana Maria Quiroz Zapata, is from Colombia. She is one of many migrants who had come to the United States illegally but received court orders prohibiting deportation to their home countries because of fears of persecution. As a workaround, the Trump administration has been deporting people to third countries.

Ms. Zapata had challenged her deportation, but the Friday ruling puts her case on track to be dismissed. Ms. Zapata’s family declined to comment.

A message seeking comment from the Department of Homeland Security was not immediately returned.

The dispute centered on whether an April 14 letter by the Congolese Interior Ministry had reached top U.S. immigration officials before Ms. Zapata was flown to Congo two days later with 14 other migrants.

Ms. Zapata has diabetes, hyperlipidemia and hypothyroidism, and the letter said that Congo would not accept her because it would not be able to adequately provide her medical care.

The Trump administration argued that neither the State Department nor the United States Embassy in Kinshasa, Congo’s capital, had received the letter. The U.S. government said that it had provided a flight manifest in advance to the Congolese government, which did not refuse her upon arrival.

Judge Leon said that the letter had reached immigration officials through Ms. Zapata’s lawyer and Representative Robert J. Menendez, a New Jersey Democrat who has championed Ms. Zapata’s case.

“If plaintiff had conclusively shown that the April 14 letter had been brought to the attention of ICE before her removal,” Judge Leon wrote, “there would be no need to modify my prior order.”

Ms. Zapata’s lawyer wrote in a June 1 filing that Ms. Zapata was “dying by inches,” housed in Congo “during an active Ebola outbreak, without her insulin.”

It’s unclear what options Ms. Zapata will now have. Most of the other migrants sent to Congo have agreed to be returned to their home countries with the support of the U.N. migration agency, and many have already departed.