It was unclear Sunday why McGowan fled after police approached him and two companions in a parked pickup truck about 2:30 a.m., Mades said.
Mades said Sheriff's Deputy Eric Byers approached McGowan, who was sitting in the truck with an unnamed woman and Larry Shelton, the 19-year-old brother of McGowan's girlfriend.
Byers asked the three for identification, Mades said. None produced an ID, but instead gave Byers names and birth dates, which he attempted to check, he said.
At that point, Deputy Robert Whittington arrived as back-up - standard procedure when police are checking out a suspicious vehicle, Mades said. The truck was parked on property marked with a "no trespassing'' sign.
Whittington saw McGowan jump from the passenger-side door of the truck and run toward the creek, Mades said.
As the two officers followed McGowan to the bank of the creek, they saw that McGowan had jumped in and was wading upstream, Mades said.
In the water, McGowan ducked behind a tree. He appeared to be trying to hide something in the water, Mades said.
Shelton later told police that he and McGowan had been drinking and smoking marijuana, Mades said. Police found what appeared to be marijuana in the truck, he said.
After moving away from the tree, McGowan turned and started to swim across the creek, Mades said.
The deputies saw McGowan slip under water. After he resurfaced, they called out to him, telling him help was on the way, Mades said.
Whittington considered entering the water, but was warned not to by dispatchers, he said.
McGowan was having an increasingly difficult time staying above water when, at about three-quarters of the way across the creek, he went under and did not resurface, Mades said.
"Hypothermia must have set in," he said.
McGowan's body was recovered by rescue workers about two hours after deputies last saw him, he said.
Police are awaiting autopsy results to tell them McGowan's alcohol level and whether he had drugs in his system, Mades said. The body has been sent to Baltimore for an autopsy.
Shelton has been charged with hindering and obstructing an officer for initially giving police a false name, Mades said.
Police are not releasing the name of the woman. Mades said she gave her true name and cooperated with police.
For the woman, Mades said, Sunday's incident was a matter of "wrong place, wrong time."
She did not know either of the men, who'd given her a ride earlier, Mades said.
Mades said it's not uncommon for people to jump out of cars and run when approached by police.
In the past, Mades said some people have tried to use a body of water as an escape route, figuring deputies in uniform would have a difficult time pursuing them.
However, given the early hour and frigid temperature of the water, McGowan's actions were unexpected, Mades said.
Until the lab results are returned, it's impossible to gauge McGowan's frame of mind at the time, Mades said. McGowan said nothing to his companions before jumping out of the truck, he said.
The water generally ranges from 70 to 90 feet wide in that area of Conococheague Creek, Mades said.
While that area of the creek tends to be relatively shallow, it has deep pockets that can plunge 8 to 10 feet, according to Department of Natural Resources Police Officer Ray Harner.
The water temperature was probably about 34 to 36 degrees when McGowan went in, Harner said.
He said that when the water is that cold, it doesn't take long for a person to develop hypothermia - meaning the body loses heat faster than it can produce it.
"The body will shut down," said Harner, who said symptoms of hypothermia would include lethargy, slow reflexes and loss of circulation to the limbs.
A person would probably lose consciousness within 20 minutes of entering the cold water, he said.
Trying to swim would speed up the process, Harner said.
Because there is no policy regarding chasing suspects into water, it is up to deputies to use their judgment on a case-by-case basis, Mades said.
Following a review of the deputies' reports Sunday, Mades said he was convinced that Byers and Whittington used good judgment during the incident.
"At this point, I think the deputies acted very professionally," Mades said.
McGowan lived with Larry Shelton's sister, Tina Shelton, Mades said.
No other information about McGowan was available on Sunday, he said.
Repeated efforts to reach Tina Shelton on Sunday were unsuccessful.
Tina Shelton's mother said her daughter had dated McGowan for two years and lived with him in Williamsport.
The mother said she thought McGowan worked for a plumbing company in Hagerstown but wasn't sure.