Federal regulators on Tuesday gave Juno Therapeutics the all-clear to resume testing an experimental cancer treatment, just days aftershutting down the trial because of three patient deaths.

Juno is at work in a newfangled field of oncology in which scientists remove a patient’s own white blood cells and rewire them to home in on cancerous growths, part of the growing field of immunotherapy. The Food and Drug Administration put the study on hold last week after three young leukemia patients who had received Juno’s experimental therapy developed fatal brain swelling.

Juno blamed the deaths on an unforeseen interaction between those reengineered blood cells, called CAR-Ts, and a chemotherapy drug used to prepare patients for treatment. It proposed resuming the trial without using that chemotherapy drug.

The FDA completed the review with startling speed, apparently accepting Juno’s explanation for the deaths and approving the new protocol.

The deaths spurred widespread questions about the future of CAR-T therapies, which have led to lasting remissions for some seriously ill patients but have been tested only in small trials. The technology itself is only a few years old, and its long-term safety has yet to be established.

Novartis, Kite Pharma, and others are pressing forward with similar therapies, and while none has reported an issue with fatal brain swelling, each has faced increased scrutiny since Juno’s revelation.

The FDA’s speed in letting Juno restart its trial is notable for an agency that tends to be conservative when it comes to drug safety. Juno had said last week it hoped to get an answer within 30 days. It took just two.

Other companies have languished for months under FDA holds, which cannot be resolved until regulators sign off on deficiencies in clinical trials. Zafgen, a biotech company developing a treatment for obesity, has had its research suspended since December in the wake of two patient deaths.

Republished with permission from STAT. This article originally appeared on July 12, 2016.