4:05 p.m.: Austin police have detained three people who may have sold the packages of K2 behind the uptick of adverse symptoms that started Wednesday, officials said.

Austin police Lt. Kurt Thomas said the three people were identified as persons of interest after reviewing surveillance cameras from the Salvation Army location on Neches Street. Police will soon file drug possession or intent to distribute charges against at least two of the people detained, Thomas said.

Thomas said police don’t know whether the K2 causing the adverse reactions is coming from the same source.

3:25 p.m. update: Since 7 a.m. Thursday, Austin-Travis County EMS have treated at least 28 people who experienced adverse reactions to the synthetic drug known as K2, Austin-Travis County EMS officials said.

That’s not counting the people they treated for adverse K2 reactions Wednesday night, which is when medics started seeing a dramatic uptick in cases, officials said.

"I don’t have an exact number (of patients) but I’m sure our department is going to get something out like dozens and dozens (of cases) between the last 48 hours," EMS Cmdr. Mark Karonika said.

Officials said they will release the total number of K2 users they treated Thursday evening.

Several ambulances have staged at several areas across downtown Austin to respond as quickly as possible to the number of calls they’re getting, EMS Cmdr. Mike Benavides said. EMS set a command post to treat patients outside the Austin Resource Center for the Homeless, at Seventh and Red River streets, which resulted in several lane closures in the area.

WATCH: Facebook Live from the scene near Seventh and Red River streets Thursday afternoon

Medics are sending patients to different hospitals "so we’re not inundating one hospital with all these patients," Benavides said.

"We’re running it like a mass casualty incident right now," Benavides said.

It is unclear what the condition of the patients treated Wednesday and Thursday is.

Karonika said the number of K2-related cases was reminiscent of the influx of overdoses last year, but couldn’t say what had caused this recent rash of cases.

"I can’t tell you where it came from," Karonika said. "What I can tell you is that in EMS, law enforcement and fire, when there is a new drug that comes out and the potency changes or whatever, we’re generally the first to find out."

Earlier: Austin police have shut down the intersection at Seventh and Red River streets while they assist Austin-Travis County EMS as they respond to a spike in K2-related cases that began Wednesday and has affected 21 patients Thursday, officials said.

EMS Capt. Darren Noak said the uptick started Wednesday afternoon and continued into the evening, though he could not immediately provide details on the number of patients treated Wednesday. On Thursday, emergency crews started responding to K2-related cases about 7 a.m. The most recent patient was treated about 1:40 p.m., Noak said.

Some of the patients were found unconscious, experiencing seizures on in altered states of mind. Noak could not say whether any of them were in critical condition.

The cases have concentrated in the downtown area, in a quadrant within Tenth Street to the north, Fifth Street to the south, Interstate 35 to the east and Congress Avenue to the west, Noak said.

Several people near the Austin Resource Center for the Homeless, at the intersection of Seventh and Neches streets, told the American-Statesman Thursday afternoon they had seen a group sharing the same joint of K2 earlier in the day. The people in the group started collapsing one after the other soon after smoking the joint, said the people outside the ARCH, who declined to provide their names. Later in the day, they said, a second group shared another joint and many of them also collapsed.

Noak said the uptick in cases in the last two days is one of the largest they’ve responded to.

"We’ve had spikes before and we’ve continued to see K2 cases, but not to the volume we’re experiencing right now or have experienced before," he said.

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Synthetic cannabinoids — commonly known as synthetic weed, K2 or Spice — are drugs marketed as a legal alternative to marijuana and, authorities say, have been sending many of Austin’s homeless people to the hospital. Last year, paramedics treated hundreds of people on the streets of downtown for K2-related symptoms that have ranged from hallucinations, seizures and violent behavior to low blood pressure mixed with low heart rate, a deadly combination.

Noak urged people not to use the dangerous drug.

"It’s very dangerous and we encourage everyone to stay away from the use of any synthetic drug," he said.